Quote fromQuote fromI have a question does it depend on the difficultly u have on to see a skeleton riding a spider, like do they on spawn on hard or do the spawn on easy,normal and hard
I was on easy mode. 'Cause I'm a coward. I honestly think it's just a matter of "if a skeleton and a spider are near each other the skeleton might decide he wants a piggy-back ride".
For what it's worth, I was on Hard... but I don't think it matters.
Also, I think it spawns that way-- with the skeleton already on the spider. I watched groups of skeletons and spiders for a long time seeing if I could catch one in the act of mounting up, but it didn't happen.
They are definitely rarer than I originally thought. My first one spawned in about a minute, but it took 15 minutes after that to find another even with constant mob spawns.