Barring abuse, most of the time when cannibalism occurs in fluffies, it’s due to either desperation (ex: starving feral mare eating her foals) or stupidity (ex: microfluffy mare trying to hide her foals in her cheek pouches and accidentally swallowing them).
“True” cannibal fluffies do exist (albeit very rarely), but they are not a mutation or separate breed any more than human cannibals are.
They are just otherwise normal fluffies who happen to prefer fluffy meat and/or enjoy killing and eating other fluffies.