I'm currently making an adventure map with custom mods. My modded mobs have id's like "zssaddon.landOctorok" instead of simply "Zombie". Because of this, commands like /kill @e[type=zssaddon.landOctorok]
simply do not work, and return saying that the UUID is invalid. I have tried quotes, as well as apostrophies.
zssaddon:landOctorok
instead. It is quite hard to test it since i'd need to mod the game and that is a bit of a hassle.