Question: Look at this

![enter image description here](https://i.stack.imgur.com/k2Jw4.png)

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 ?

Answer: 
[Spider jockeys](http://www.minecraftwiki.net/wiki/Spider_Jockey) 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.