Yes, yes, I know, I know, it's the spider that comes first and the skeleton that is randomly spawned by the game to ride said spider comes second when a spider jockey is spawned, but the skeleton is still spawned nonetheless. So, in Java edition, during a full moon, hardcore mode, maximum regional difficulty, on a superflat world, made up of a 1 block thick layer of a solid block, like, say, obsidian, what is the probability that any given skeleton was spawned as part of a spider jockey?
The chance of a spider jockey is fixed at 1%, regardless of any factors:
public IEntityLivingData onInitialSpawn(DifficultyInstance difficulty, @Nullable IEntityLivingData livingdata)
if (this.world.rand.nextInt(100) == 0)
EntitySkeleton entityskeleton = new EntitySkeleton(this.world);
entityskeleton.setLocationAndAngles(this.posX, this.posY, this.posZ, this.rotationYaw, 0.0F);
This is from the source for 1.12.2, the latest version that I could find source for online but I doubt it has changed since (it is also the same in 1.6.4, and likely since they were added in Alpha 1.0.17).
If you mean out of any naturally spawned skeleton, including skeletons not spawned as part of a spider jockey, then the chance is 0.99% since spiders and skeletons have the same base spawn probability (for every 100 spiders there will be 101 skeletons, one of the latter of which will be part of a spider jockey).