What's with this skeleton riding a spider?
Solution 1:
Spider jockeys have been in the game since seecret update 8 of Alpha. They are extremely rare, but not unknown or undiscovered.
Solution 2:
As the answer by Dave McClelland says, its a spider jockey. A spider Jockey possesses the power to climb and run quickly like a spider but can shoot arrows like a skeleton, making it difficult to defeat. of all the spiders that spawn in minecraft theres only a 1% chance that it will be a spider jockey.