If you put a spider spawner in the nether it has a .8% chance of occurring.
 But just imagine.
You're running in a nether fortress all closed in, you see a staircase and a little bit of nether wart peaking out.
You're glad you finally got what you needed for your potions, now you can go back and make the next set of awkward potions.
But what is this?
It's a chest that you forgot to loot.
Maybe there's some diamonds in there!
You come up to the chest without looking off to the right where the corridor continues on.
Three diamond and diamond horse armour and an iron bar. Not bad loot.
As you shift click everything into your inventory you hear a scuttle near you.
A spider? How strange, they weren't supposed to spawn down here.
You quickly take down the chest menu and equipt your trusty diamond axe.
As you turn you see the face of a wither slightly higher than it should be. And then you realize not only had you not heard that with but it is on top of the spider!
You try to battle the hell-spawn but before you could even un-mount this terrifying jocky you succumb to the withering effect.


Side note lawrence if any mob can ride any mob then you absolutely must add a Creeper/charged Creeper spider/cavespider Jockey to one of the Mob Arenas!

well, if u ever find one, run for your life
but as Lawrence mentioned I don't think that's possible, and if it is itd be very rare

If you put a spider spawner in the nether it has a .8% chance of occurring.
So not possible until we get our own spawners. So never =(
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not on Noobscraft atleast