Whilst riding a mob, it is immune to freezing damage.
Steps to Reproduce:
1. Spawn a mob that it rideable.
2. Go inside powder snow and ride the mob.
3. You will notice you start to freeze but the mob you are riding on doesn't.
The player freezes but the mob you are sitting on doesn't.
Both the player and the mob you are sitting on should freeze.
I don't feel the need to attach screenshots or videos as I feel this is quite self-explanatory.
MC-205104 Rideable mobs are not affected by powder snow's freezing effect while being ridden