a.) If you are less than 11 levels above the level of the dungeon/raid then personal loot rules are in effect.
b.) If you are 11 or more levels above the level of the dungeon/raid then Legacy loot rules are in effect.

https://us.battle.net/forums/en/wow/topic/20761866700

If personal loot is in effect then there is a chance (albeit very small) that everyone could get a pet, in legacy loot I don't think you'd get more than one. If you are running a legion raid thats still personal loot not considered legacy.