Mob that climbs walls (where's "entity collides with block"?)

I want to create an enemy that climbs walls. I've looked through the forums, and I've been told one of the methods is using a trigger, "When entity collides with block." At least I assume it's a trigger, because I can't seem to find it in the triggers. Any way to create a mob that climbs walls? (sorry if this is hard to read)