Catalpa worms

Have NEVER known anywhere to buy them. Don't see them on trees either but there are only a couple that I know of and occasionally look at.

I keep seeing these pop up on the OB, but what exactly are catalpa worms. Googling it just made it look like they're caterpillars? Or are they specifically the little green caterpillar looking things. Because I rarely see those, but I see a good bit of caterpillars