What is a good mid-game pet?

Typically I'm a human lawful valk (good starting class as I can't really get past mid-game yet), and I'm curious if I should consider switching (or adding) pets.

I'll usually level my cat/dog up decently well...letting them take the majority of the kills and so on to support me in the early game. When I get to the mid-game (sokoban, lower mines, quest, etc) I seem to die...quite often, and I'm wondering if a change of pet would be beneficial for me.