I put my two bins outside for the summer and inherited a lot of these guys. What are they?

Horned caterpillar & frass. Too beautiful to kill. So he's living in a plastic ventilated box on my sunroom.

He's been in captivity for only about 3 hours but already he's pooped up a storm. Lovely little greenish black blossom-pellets, called "frass." Remember that for your next game of Scrabble.