Just to get it to 125 Dex first, as the others say.
However, I make it a point to work the pet against things that will give it skill gains when going for the dex gains, as there's no reason to waste the gain opportunities. For example, I tend to take my 1-slot beetles to one of the umbra small orc forts to train dex up, where it can get gains for wrestling, tactics, anatomy and resisting spells at the same time. I just have to occasionally vet or cast greater heal on it.