Identification Help!

I've been a gardener for about five years and this is my first pepper mix up. I'm growing rainbow bells, thais, jalapeños, and another sweet bell medley. What the heck is this guy? Where did he come from? His plant is popping like crazy and I have no idea what he is.

Flavor is sweet-ish with a very peppery (but not spicy) aftertaste. Skin is very firm but not exceedingly thick. It's about as long as a small hand.