Quote from Reptile;954304:
I'm pretty sure Duy said only rangers can choose a pet in the beginning but every class can later on, but I found juvenile drakehounds and it won't let me charm them.
No, I don't have any other drakehounds, I don't have any pets on my thief, and pressing K does nothing.
Only Rangers can tame pets to be used in combat. I think you're thinking of miniatures, which are cosmetic pets that are mini-sized versions of the many NPCs and enemies in the game. You get those by buying them in the Black Lion Trading Post, or from the market since people sell them.