Has anyone seen a plug like this before? It is supposed to be for a portable cooler. Wondering what it plugs into?
That would be a NEMA 6-15 appliance plug. It's different so you don't plug an appliance expecting 220v into an outlet delivering 120v, which would multiply the current to dangerous levels. 

Apparently they're also used in 12v (RV) applications sometimes. 

See https://diy.stackexchange.com/a/89242/35141.