I'd turn off box main, take out ones on either side and see what
gives...
...
It is a breaker the size of 4 thin ones and right at the bottom of the
panel. I took out a thing 15 amp above it and I still can't get it
out. ...

I could/did remove the entire face plate of the panel, just leaving the
breakers in place. Perhaps its "hung" on the panel face. At any rate,
you'll get a better 'look' at it after you take it off.

Use normal safety measures