What is the RAREST thing to spawn in Minecraft?

These are things like, Spider Jockeys and Baby Zombies riding regular zombies and stuff like that. But, I want to know the most rarest thing to spawn in Minecraft naturally.


Rays Works has an excellent series about rare mobs. Here is his latest video on the new rarest mob:

It happens to be a strider riding a strider, riding a strider, etc. with a final baby zombie piglin on top, with a Jack-O-Lantern head.