Bought a BMW Mini fuse box off Ebay, cheap source of some decent quality relays and blade fuses, also had these "things"? Guess they are some kind of fuse
I don't recognise them. If they have two terminals and low resistance between them, they might be fusible links. Do they have '50' written on them? That might indicate a 50A rating.
They are J-Case fuses, very widely used on modern cars. They have a longer time delay than blade fuses, so more suitable for loads with high surge currents.