Minecraft wiki entry for Summoning_jockeys, associated text: A spider jockey has a 1% chance of spawning out of every spider or cave spider‌[Bedrock Edition only] spawned, and in the Nether, there is an 80% chance of a spider jockey becoming a Wither skeleton jockey, in snowy biomes there is 80% chance of cave spider jockey with stray rider. In Java Edition Baby zombies have a 5% chance of spawning on a chicken and an additional 5% chance if there are no chickens in a 10x6x10 area, while in Bedrock Edition, baby zombies, husks, or zombie villagers has 15% chance to become jockey when try to attack player, villager, wandering trader, or golems and can ride more mobs (not just chicken). For a skeleton trap to spawn, there is a small fraction of a chance they will spawn under a lightning strike in a group of 4 (0.75–1.5% chance on Easy, 1.5–4% on Normal, and 2.8125–6.75% on Hard, depending on regional difficulty).