What's with this skeleton riding a spider?

Look at this

I've found a skeleton riding a spider in my world, could it be a hidden monster ?

It was very bugging jumping up and down from the spider when the spider died quickly from another monster around.

Any explanation ?

2 Answers

Spider jockeys have been in the game since seecret update 8 of Alpha. They are extremely rare, but not unknown or undiscovered.

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.