* MegaScientifical makes a base inside a mountain, entrance on top... Creepers drop from above somehow, without anything above... Result:

Not to mention, literally like 3-5 Creepers attacked me during being in that, which I'll add was laggy as forget.
Edit: Someone theorized on Minecraft Wiki that if you use the Spider Spawn Egg enough, it should eventually make a Spider Jockey because of how they are normally spawned... I'm having trouble believing this applies...