This might be related to the spawn rates in general, but I noticed that when skeleton horses spawn during a thunderstorm, they no longer trigger the four horsemen when you approach them.

How to repeat:
-wait for a thunderstorm
-find a skeleton horse
-run up to the horse

Expected result:

• the trap triggers, and the four horsemen of the apocalypse descend upon you with murder in their eyes

Actual result:

• the trap fails to trigger