The bug
Slime and magma cube mounted rideables mobs need to be tamed, inconsistent with other mobs.
Does not affect 1.20.1.
Steps to reproduce
Use the following command:
/summon minecraft:horse ~ ~ ~ {Passengers:[{id:slime}]}
/summon minecraft:horse ~ ~ ~ {Passengers:[{id:magma_cube}]}
Noticed that slime or magma cube was thrown off by an untamed horse.
Use the following command:
/summon minecraft:horse ~ ~ ~ {Passengers:[{id:husk}]}
Noted that rideables mobs ridden by other mobs do not need to be tamed.
All affected rideables mobs:
Horse
Donkey
Mule
Skeleton horse
Zombie horse
Llama
Trader llama
Confirmed!