Skeleton horse traps do not activate when approached. If you exit and reload the world, the skeleton horse trap will activate, and several skeleton horses and skeletons will appear, but only one skeleton will be riding the trap-horse, and all the other skeletons will not spawn riding the skeleton horses which also spawn. Exiting and rejoining the world and approaching that same horse will cause the trap to activate again with the same outcome as before, with some riderless skeleton horses and mountless skeletons spawning. This process can be repeated indefinitely.
Steps to reproduce:
1. Create a superflat world with cheats enabled.
2. Use the command "/weather thunder"
3. Find a skeleton horse trap and approach it. Notice how nothing happens.
4. Exit and rejoin the world.
5. Approach the skeleton horse trap again. Lightning will strike and several skeletons and skeleton horses will spawn, but none of the skeletons will be riding the horses... except for the trap-horse, which will have a skeleton rider.
6. Repeat steps 4 & 5. You will end up with tons of skeleton horses and skeletons.
Note also that the skeleton on the trap-horse cannot control it. See
Also note that the weather reverts to being "rain" when relogging. This is probably related to
Whenever I go up to a skeleton horse trap in creative it does not work