Lake Malawi rock dwelling cichlids. Especially when you end up with just one because it killed the rest and it just hides in the rocks all day waiting for you to add a new cichlid for it to kill.
Upside down catfish. Stays in the cave all day. If i turn off the lights they still won't come out. I think I'll have a better chance of seeing the Lochness monster than seeing them swim upside down. But I'll keep 'em, if I buy a fish I never return or rehome them no matter how boring they are.