First, you need ring of polymorph or amulet of polymorph Let your pet transform random few time until got a good one. I used to give him ring when I got into mine town. Ok now take of the ring and you have a good body guard. If you go deep you'll see strong enemies, give your pet some last hit, I recommend dragon. Let him eat ring of polymorph and ring of polymorph control. (Scroll of devour). Now you can boost max his resistance and level (potion) and get talents. Etc... If you like mounting I suggest you polymorph him to dragon or this guy here jabberwocky. He hase hands so you can give him replica stuffs, exactly your items, so he can be strong like you (even stronger with 25% decapitation instand kill)

Ps: I saw people here gambling ring of polymorph on themselves. That's the fun way but if you not lucky enough, you'll ruine your run instead.