This is the rarest mob spawn according to every site unless that changed. But this is what I found on my island at night.

 

That's not that rare, I've had zombies with amour on a chicken before...

 

It was my first time ever seeing one.

 

I've seen heaps of mini zombies riding chickens on my island, not that rare. Just uncommon

 

Haha, I love the way they look.

 

@Pile_of_Butts @Spaggy RAREY

 

He looks suu cute :>

 

Hey that's me messaging you.

 

You don't tag me ...

 

Rarest is a spider jockey with a wither skeleton rider. Next rarest is a spider jockey with a regular skeleton.
However according to the math, on Halloween the rarest possible in the game is a spider jockey with a skeleton rider wearing a pumpkin.

But that little guy is adorabr!

 

What about rabbits? They are pretty rare

 

Considering that in single player passive mobs do spawn but its a low, low spawn rate.
(from Minecraftforum: 15) Build a platform 24 blocks away from the island, for mobs to spawn.)

 

What about spider jockeys? They're pretty rare .

 

Said it a few post up.