I found a youtube video where they said to feed them exquisite meat, but that just gives my greater mi-go +7% on agility and vitality. Then i saw a video on pets in general that said the first food in a pet's preferred diet (that you can see when you've got their backpack open) = strength food. For my greater mi-go, that's human flesh. So i killed and butchered some humans, and loaded my pet up with 4 stacks of dismembered npc flesh. Went and did some more killing and even after a level up, the greater mi-go's stats sheet is still showing +7% on agility and vitality.
Does anyone know what food(s) will actually help the greater mi-go gain strength? i'm reluctant to keep experimenting and wasting levels, this one i've got is black and red and already has 100% on vitality.
Edited to add the answer: it *is* raw exquisite meat for 14% strength, i think my ui must just have been stuck showing agility and vitality. i took a second greater mi-go out and gave it only exquisite meat in its first inventory slot and it showed +str. took everything off the original pet and only gave it exquisite meat and whilst the ui was still showing +agility and vitality, after levelling up again it started doing +str.
edit again: also make sure the buff food is in the pet's first inventory slot.