The most popular theory is that Smithy created them. It is well known that he had the technical capabilities to build such a machine, and most of his army was made up of mechanical monsters. It is also important to consider that the robotic dogs first appeared during his invasion and mostly disappeared afterwards.
Some argue that others, such as Bowser and Booster have enough technology to build something like a K-9, and may have been responsible. They also point out that Smithy sent an organized battle unit straight to the Mushroom Kingdom Castle, so why would he need the basically wild K-9s? Perhaps he sent them to round up and organize wild monsters into fighting units.
Most Mushroom Kingdom experts have come to agree that Smithy is the most likely candidate for building the K-9s, so they are considered part of the Smithy gang. K-9s are not very dangerous on their own, but they usually team up with stronger monsters.
"These cool canines in their hip shades would like to take a bite out of you. You can limit the damage from K-9's Fangs Attack by pressing the A button just before one takes a chomp. Unfortunately, there's no defense against K-9's spine-chilling Howl Attack, which weakens your attack capability." - SMRPG Player's Guide
Attack Pattern: They have three attacks: The Fang Attack is a more powerful version of their regular one, and the Howl attack will scare your characters, and cut their offensive powers in half.
Basic Strategies: I find that it's a good idea to beat K-9s before fighting anyone else in the enemy party. They have strong offensive attacks, but are relatively easy to beat, so taking them out early can make the rest of the battle easier. Mario can beat them with one timed-hit. Have Mallow soften up other enemies while Mario quickly defeats the K-9s.
Advanced Strategies: You might be able to beat all of the K-9s in an enemy party with Mallow's Thunderbolt attack, which is worth considering, although they aren't that hard to beat with regular attacks, and a single Thunderbolt may not be enough. Blocking involves pressing the A button just as they "chomp" at you.
