Various characters, with various means and sources, have done some checking into the creatures called 'goblin-rats', and pooling their results and disregarding the more obvious inconsistencies, have come up with the following which they believe to be fairly reliable information.
Goblin-rats are natural shapechangers whose bloodline is carried by a small fraction of the bakemono population, possibly linked to whatever blood results in some bakemono having significantly better (i.e. ~10 instead of ~3) intelligence than the majority. Goblin-rats do not transmit their condition by any means other than through breeding - and the condition doesn't necessarily occur in every generation either, so most goblin-rats are born to 'normal' bakemono, generally with at least one 'smart' bakemono for a parent. Their condition manifests itself at puberty. Such bakemono usually leave their kin to seek out others like themselves and form their own packs, though a few do remain in their bakemono packs and become warlords themselves. (Indeed, it is possible that some goblin-rats get slain in bakemono form and their true nature is not even realized by their slayers.) As shapechangers, they are not affected by spells which target humanoids but are vulnerable to those which target shapechangers instead. They are still goblinoids as well though.
Goblin-rats are deathly afraid of felines. When confronting same, they have to overcome that fear (Will save) or be panicked. A masterwork sculpture or painting crafted for that specific purpose can protect a place from being bothered by goblin-rats.
Goblin-rats have three forms. Their base form is that of a bakemono, and they revert to that form when slain. Their other two forms are that of a dire rat (about 2' long or so in the body) and a hybrid form from which they take their name, with the head, torso and tail of a rat and the arms and legs of a bakemono. Like many other shapechangers, they heal as though from a day of rest when they change form - it isn't known whether they have a limit on how many times they can change forms in a single day. Unlike hengeyokai or use of a polymorph spell, however, they do not absorb equipment into their dire rat forms, so often they end up leaving equipment at home, even though as the equivalent of warlords they may be proficient in all sorts of weapons and armor.
In bakemono form, they have no special immunities - but they are very difficult to distinguish from normal bakemono - a ranger with either goblinoids or shapechangers as a favored enemy may do so (Sense Motive, DC 15) but others find it much more difficult to tell from observation alone. However, they are definitely superior to bakemono - thanks to both their rat-like instincts and their shapechanger nature, they have slightly increased natural armor, improved saves, and heightened senses (in the form of a racial bonus to Listen, Search and Spot skills), while still enjoying the natural weaponry of their bakemono form. They also have the ability to communicate with rats empathically.
Goblin-rats advance in levels as bakemono warlords do - most are warriors, a few may advance as adepts (worshipping the Lord of Rats), and even fewer advance as rangers, rogues and shamans. Like bakemono warlords, they will often use armor (sometimes including shields) and weapons, either small weapons in one hand or medium weapons in both hands - but as they are sometimes caught without equipment (having been in rat form recently for scouting or whatever) they may simply choose to fight with their bite and claw attacks. Most are fairly low level, but high-level goblin-rats are certainly possible.
((GM digression - when determining NPC levels, I often use an 'open-ended' roll, where the highest result means add 'N-1' and roll again. SO say I was using a d4 to determine a goblin-rat's level, a 1-3 would mean a 1st-3rd level goblin-rat, but a rolled 4 would mean 'add 3 and roll again', so a 4 followed by a 1-3 would mean 4th-7th level, a 4 followed by another 4 would be 6 levels and another roll, and so on. Yep, it IS possible for the GM to generate a creature that can whup PC butt - and it would be up to the PCs to figure that out or deal with it if they have too... But even though the 'add and roll again' results are more likely the smaller the die size, the critter is still better off rolling a larger die size for levels, and the base die size is what I have incorporated into my tables, such as they are. A d4 doesn't rate to get up to too high a level in the end...))
In dire rat form, they are also very difficult to distinguish from dire rats by observation alone, though in the dark their eyes do glow red. They are fairly stealthy and quick in dire rat form, which they use primarily for scouting and being inconspicuous, generally amongst a pack of normal dire rats. They gain some advantages in this form, including some increased physical ability (DEX and CON) scores, though they have only one effective attack (bite) and lose much of their natural armor.
In hybrid (a.k.a. goblin-rat) form, their nature is clearly obvious to anyone observing them - but it is in this form that they combine the advantages of both other forms to a great extent. They can still attack with a bite and two claws, and although their bite may not be as dangerous, they are more adept with both bite and claws, or they may choose to fight with a weapon - while also getting a bite in every now and then. They have all the skills of both forms, the excellent natural armor of the bakemono combined with the agility of the rat, and so on...
Their most notorious ability in both dire rat and hybrid forms (but not in bakemono form) is their ability to resist damage from mundane weapons, as their wounds essentially heal up instantaneously from such weapons (unless the wounds are very deep, of course, in which case they only partially heal). Magical weapons of +1 or greater enchantment deal normal damage to a goblin-rat in non-bakemono form. In addition, weapons of jade or obsidian, even if non-magical, can affect goblin-rats, much as they can affect Oni and similar creatures. However, making melee weapons of jade or obsidian (O.A> p. 142) is both expensive and inefficient in terms of quality, and the cheaper of the two materials, obsidian, has other dangers when used in melee against creatures it is effective against. On the other hand, wounds dealt by jade and obsidian weapons to goblin-rats do not heal normally and are rather painful to them. There are a few cheap solutions...
First, of course, is to buy magical ammunition for missile weapons. It costs the same to enchant 50 arrows as it does one melee weapon - the cost of 50 arrows +1 is 350 (50 masterwork arrows) + 2000 = 2350 - an average price of 47 gp per arrow when bought in such bulk. When bought in smaller quantities, the price increases somewhat - the market will sell +1 arrows or other ammunition in sets of 10 of the same type, for 500 gp.
Secondly, a character may buy flasks of 'jade powder' (O.A. p. 78) - each flask is enough to properly coat one melee weapon, and the coating will last until a successful hit is scored. These cost 100 gp each, and are as described in the book. One advantage 'jade powder' has over an actual weapon of jade is that the weapon coated with it has its normal to hit and damage bonuses, whereas weapons actually made of jade or obsidian are inferior in that regard.
Third, one can buy even cheaper ammunition which is effective in a different way - obsidian-tipped arrows and bolts (but not sling bullets) can be acquired for 10 gp each. As missile ammunition, they don't have the associated danger that wielding an obsidian weapon in melee has, and they do cause wounds which do not heal normally. The obsidian tips are very fragile and thus the arrow is 'destroyed' when it hits and has (like other arrows) a 50% chance of being destroyed when it misses. An obsidian-tipped arrow is -1 to hit (-2 for obsidian material but +1 for masterwork arrow) and -2 to damage (minimum of 1 point of damage), so in many ways it is inferior to magic arrows. But since one can get 5 such arrows for the price of a single +1 arrow - and since such arrows affect a variety of creatures, not just goblin-rats - this is often the path chosen by border wardens and 'shapechanger hunters'. (Obsidian is much cheaper than jade, and since both materials are fragile, especially in the size and shape of an arrowhead, nobody makes jade arrow tips for general sale...) Obsidian-tipped arrows are available in Marike singly, by the handful, or by the score, they cost 10 gp each regardless of the quantities bought.
It is believed that when in non-bakemono form, a goblin-rat's bite is as filthy as those of rats and dire rats, i.e. is quite capable of spreading filth fever.