Depends on what type of pet, if it is a mount or just a regular pet
reg pet your main ini files would be
3dmotion
3dobj
3dtexture
3DSimpleObj
3DSimpleRole
armor
DynamicRank
EudRbnType
than also if you wanted to be able to re egg add to
ChangeEgg.ini need to do db side also
PetDescribe pretty self explanitory lol
EudBook
EudBookDes
Mounts you would also have to edit the following files
Mount
MountMotion
mounttype
I think i got them all lol hope it helps