I had rollmops for lunch today and decided to look into the origin of the name. A rollmop as you probably know is a pickled herring fillet, rolled up and usually held together with a short wooden skewer. I got the roll bit but the mops was a mystery.
I looked it up in the OED and it turns out it's a 19c borrowing from German Rollmops, which is Roll + mops, meaning 'pug', so Rollmops literally means 'roll pug'. The plural is Rollmöpse, Rollmops being the singular, and English rollmop a back formation.
This is a bit like 'pea', which is a back formation from 'pease', as in pease-pudding. 'Pease' sounded like a plural and so people assumed one of them would be a pea.