The spider jockey is not a mod. It is nor a joke. It is a very rare combination of a spider and a skeleton.
It is a very interesting creature, as it can survive after one dies.
FROM MINECRAFT WIKI:

An advisory on using wolves

Spider jockey battles involving wolves are slow and messy at best and fatal to everyone but the spider jockey at worst. Wolves are not recommended as a method of defense against one since they will not attack it as a whole - they will aim for either the spider or the skeleton, depending on which the player hit or was attacked by. Conversely, when a wolf attacks a spider jockey only the individual it managed to injure will try to go after it. Once hit, a wolf will focus its hostility on the mob that last attacked it. Since a skeleton's ranged attack makes it more likely to hit the player or a wolf, it will often be the first targeted. The wolves will then have a harder time attacking the skeleton because of its lofty arachnid perch, causing them to crowd around the spider in a futile effort to get at the skeleton while the spider continues to injure the player.

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A spider has a 1% chance of spawning with a skeleton riding it.

Ive only encountered one once. If possible, send me the images from your minecraft if you find one!

(I didnt make the video)