What is this bump thingy?

Appears to be the result from some type of trauma and is probably in the process of healing...sometimes they heal completely and sometimes they don't. You didn't mention that when you poke at it, if your horse responds in a negative way. It could be anything from an insect or horse bite to a kick or simply horseplay. If the vet is coming anyway and it won't cost you anything to be sure, I would take advantage of the opportunity to know for sure.