This happens when I am experimenting a monkey raid farm with about 4 Houndiuses. But they usually start to attack each other when fighting the monkies.
Logging out stops them from killing each other.
I can provide a video later if it's needed.
1/Put about 4 or more Houndiuses near each other
2/Summon the raid
3/Watch as some Houndius may start to attack other Houndius during the raid.