I just got two skeleton traps spawned in one thunderstorm on my Hardcore world... the second batch spawned right after I killed their riders... (one of the horses, unfortunately, died tho) :
Minecraft | Lightning and Skeleton Trap Farm!! | Vanilla Survival | Fixed in 1.13 - YouTube
Now in Minecraft 1.9, summon the 4 horsemen (skeleton trap) | Summoning, Minecraft, Minecraft 1