As anyone who has read the J.K. Rowling's Harry Potter series will be aware, hippogriffs are introduced in book three of the series, The Prisoner of Azkaban:
Rowling also describes the species under the pseudonym of Newt Scamander in one of her two Harry Potter "schoolbooks" (written to raise funds for charity) Fantastic Beasts and Where to Find Them:
It is puzzling to note that nowhere in these descriptions does Rowling indicate that hippogriffs, as created by Ludovico Ariosto (discussed in History and Origins of the Hippogriff), are in fact half horse and half griffon, rather than simply horse-eagle. In fact, her description of them in Fantastic Beasts as egg-laying creatures would seem to contradict the traditional version of hippogriff ancestry. While it is perfectly normal that Rowling take liberties with myths and reshape them to suit her story-telling needs, there is also the possibility that she is not aware of the hippogriff’s mixed griffon/mare parentage.
Rowling has not, in her many interviews, clearly stated the source from which she drew inspiration for her version of hippogriffs:
Transcript: Press Club October 20, 1999
The Boston Globe October 18, 1999
"All about Harry Potter from Quidditch to the future of the Sorting Hat"
by Stephanie Loer
Harry Potter and the Prisoner of Azkaban DVD "Extra," 2004:
ALFONSO CUARÓN: There are not that many graphical representations of hippogriff, and that is something with the story that is very interesting. There are sphinx, there are several sphinx, or you see creatures that are half bird and half cat, a lot of different things. But for hippogriff, it was actually hard to find....
JO ROWLING: I knew that 'cause I went looking.
ALFONSO CUARÓN: You knew that, yeah.
JO ROWLING: I could hardly find any anywhere.
ALFONSO CUARÓN: No, I know.
JO ROWLING: So I thought it's complete liberty to invent.
Upon first coming across these interviews I was rather baffled by J.K. Rowling’s comments regarding hippogriffs. Rowling conceived of and began writing the Harry Potter series before the internet existed, so it’s safe to say that her familiarity with hippogriffs has no relation to online sources, which means she has to have first come across the creature in her reading. However, her comments in interviews have led me to believe that she had not read Orlando Furioso, in which, inspired by a few lines of Virgil’s Eclogues, the hippogriff was first conceived of by Ariosto. She seems unaware that the creature in fact originates from the Renaissance period (her comments give the impression that she believes it to date back to Medieval or Classical times) and that the creature comes from the mating of a gryphon and a mare, rather than simply being a horse-eagle composite.
As one can see from the above interviews, Rowling refers to the hippogriff as a "mythical creature" and says it "exists in folklore and mythology." Furthermore, in answer to a question asking why the creature is part horse and eagle, she concludes with "I don’t really know." She also suggests that people once believed hippogriffs really existed. Now the hippogriff originates from a Renaissance poem, so it’s clearly not a mythological creature . I also find the word "folklore" rather inappropriate since folklore entails traditional beliefs or tales and this cannot be applied to the creation of a sixteenth century poet . Furthermore, though Rowling states that "Medieval European people genuinely believed it existed," I am not convinced anyone ever actually believed hippogriffs to be real creatures. As Cuarón points out, there are very few representations of the creature. That shouldn’t be a surprise if one accepts that it’s the creation of a single poet, rather than something that comes down to us from multiple tales and Medieval bestiaries as the griffon does. To my knowledge the hippogriff does not appear in Medieval bestiaries, in contrast to its forefather the griffon which does and which has a far more extensive history and was, in all likelihood, believed to be real at a certain point in time.
My theory for Rowling’s source for the hippogriff is that she is familiar with it through the hippogriff episodes from Orlando Furioso as recounted by Thomas Bulfinch in his Legends of Charlemagne or Bulfinch’s Mythology, a compendium of several volumes of Bulfinch's work including Legends of Charlemagne . Bulfinch’s re-telling of classical myths and Medieval and Renaissance tales is often far more palatable than the originals. Bulfinch’s Mythology is a common text in public and university libraries and thus easily accessible, so it’s plausible that Rowling would be familiar with it. It would also explain why she seems unaware of the hippogriff’s connection to the griffon. There is indeed an important piece of evidence to suggest that this is the case...
If one compares Ariosto’s and Bulfinch’s account of the scene in which the hippogriff first appears, one can see that Bulfinch’s version is quite faithful to Ariosto in this section— except for one slight detail. Bulfinch describes the hippogriff, but leaves out the fact that it comes from the mating of a gryphon and a mare!
Here is a prose translations of the relevant passage from Orlando Furioso:
Now compare the above with Bulfinch’s version:
As to the winged horse, there was no enchantment about him. He was a natural animal, of a species which exists in the Riphaean mountains. Like a griffin, he had the head of an eagle, claws armed with talons, and wings covered with feathers, the rest of his body being that of a horse. This strange animal is called a Hippogriff.
In Bulfinch's account, the line about griffons mating with mares isn't there!
So, though Bulfinch is by and large faithful to the original, he leaves out the line which states that the hippogriff is the product of the mating of a griffon and a mare— just as J.K. Rowling does. As this would explain Rowling's apparent confusion about the origins of the hippogriff, I think this is compelling evidence that Bulfinch, rather than Ariosto, was Rowling’s source for the hippogriff. 
Copryight Erika Lachapelle, 2005.
 J.K. Rowling, Harry Potter and the Prisoner of Azkaban (Vancouver: Raincoast, 1999) 88.
 Newt Scamander [J.K Rowling], Fantastic Beasts and Where to Find Them (Vancouver: Raincoast, 2001) 21.
 J.K. Rowling, 20 Oct. 1999 Press Club. 19 Feb. 2005 http://www.the-leaky-cauldron.org/quickquotes/articles/1999/1099-pressclubtransc.html
 J.K. Rowling, Interview with Stephanie Loer. 18 Oct. 1999. All about Harry Potter from Quidditch to the future of the Sorting Hat. The Boston Globe. 19 Feb 2005. http://www.the-leaky-cauldron.org/quickquotes/articles/1999/1099-bostonglobe-loer.html
 J.K. Rowling, 23 Nov. 2004. Interview with David Heyman, Steve Kloves, Mark Radcliffe, Alfonso Cuarón, and J. K. Rowling. Harry Potter and the Prisoner of Azkaban DVD "Extra." 19 Feb 2005. http://www.quick-quote-quill.org/articles/2004/1104-poadvd.htm
 Mythology according to 10th edition of the Concise Oxford Dictionary, consists of "a collection of myths, especially one belonging to a particular religious or cultural tradition" or "a set of widely held but exaggerated or fictitious stories or beliefs." Both definitions suggest that mythology entails belief. It is, however, unlikely that anyone ever actually believed in a the hippogriff due to its relatively recent origins, thus making it inaccurate to describe the creature as "mythological.""mythology," The Concise Oxford English Dictionary, 10th ed. 1999.
 Much like "mythology," "folklore," as defined by the 10th edition of the Concise Oxford Dictionary, suggests belief: "the traditional beliefs and customs of a community passed on by word of mouth." And again, as the hippogriff was created by a single author in the sixteenth century, it is unlikely people ever believed it to be a real creature."folklore," The Concise Oxford English Dictionary, 10th ed. 1999.
 Thomas Bulfinch (1796-1867), born in Boston, Massachusetts, he studied at Harvard worked as accountant at the Boston Merchants’ Bank. Legends of Charlemagne, or Romance of the Middle Ages was published in 1863. For more info on Bulfinch see here.
 Ludovico Ariosto, Orlando Furioso: an English Prose Translation. Guido Waldman, trans. (Oxford: Oxford UP, 1974) 32.
 Thomas Bulfinch, Legends of Charlemagne, Bulfinch’s Mythology (New York: Gramercy Books, 1979) 719.
 As for the quote in which Rowling mentions that she’s seen medieval illuminations of hippogriffs, I would suggests that she was confusing these with illustrations of griffons or other similar chimerical creatures such as the simurgh.
To my knowledge print sources are unanimous in attributing the creation of the hippogriff to Ariosto. I have not found any reference in my research to the hippogriff in medieval art and, after scouring the web and my university library found no (reliable) evidence of pre-Renaissance origins of the hippogriff.
As for information that I (personal opinion, please don’t sue) consider unreliable there was the following in an Encyclopedia Mythica entry:
Encyclopedia Mythica: "Hippogriff" 22 Feb 2005 http://www.pantheon.org/articles/h/hippogriff.html
However, this suggestion is completely unsubstantiated. What Greek art? What Medieval legends? Its comments are very brief and do not give much explanation, whereas the written sources I consulted (please see the Bibliography section) were unanimous in suggesting that Ariosto was the creator of the hippogriff.
There was also the following:
I cannot with absolute certainty deny any of it. However, I have found little to no evidence to support such claims and a great deal that contracdicts them. Rather, I propose the Oxford English Dictionary as a more reliable source. If one looks up "hippogriff" one is able to learn the origins of the word.
Note that it gives a 17th century origin for the word. This roughly matches the timeframe in which the English translation of Orlando Furioso appeared i.e. the 1591 translation by Sir John Harington. Critics credit Harington with popularizing the work in England:
It was, in fact, largely owing to Harington’s translation and commentary that Ariosto’s poem won that reputation [i.e. the reputation as modern classic]
With the translation and popularization of the work at the end of the sixteenth century, it would make sense that the word "hippogriff" entered into the English language in the seventeenth century. Furthermore, the larger (15 volume) Oxford explicitly attributes the word "hippogriff" to Ariosto:
I think this is enough to suggest that there is some error in the above sites (as well as in many others with similar claims) when they suggest that the hippogriff is Greek in origin as well as in Rowling’s comment about Medieval illuminations of hippogriffs. The only thing I can suggest is that perhaps there was some confusion with other types of composite creatures since mythology does indeed abound with them and there were gryphon-like or hippogriff-like creatures in Middle-Eastern cultures, the Persian simurgh or simoorgh, for example.
Please see Bibliography for a complete list of sources.