Manbearcat
Legend
Tiger Woods and Michael Jordan normally play against humans.
Fighters often find themselves in battle against black puddings.
I'm not sure the same psychological rules apply.
5e should be able to trivially handle that with its "(GM) rulings not rules" ethos.
If the GM determines that their conception of the creature's neurological system/cognitive capacity doesn't comport with being demoralized, then they can just voice that...and, if need be, the Fighter can spend their next use of <Reskinned Portent> elsewhere.
(Not my solution, but...seems pretty straight-forward) Done.