Minecraft wiki entry for Development_versions, associated text: 80% of skeletons spawned above ground in ice plains biomes, and their variants, will be strays.
All spawns where the skeleton does not have a view of the sky will be normal skeletons.
Spiders spawned in cold biomes have a chance of having a stray riding it.
Acts mostly like a regular skeleton.
Its SkeletonType tag is 2.
Drops are the same as skeletons, but also have a 50% chance to drop 0-1 tipped arrow of Slowness when killed by the player. With looting, chance increases to 2×level+1⁄2×level+2.
Shoots Tipped Arrows of Slowness (0:30) at the target.