Was the concept of an Arahant unique to Buddhism or did it predate it?
I believe it’s in Jainism too. Arihant (Jainism) - Wikipedia
Jains also have it, maybe the matted-hair ascetics (Jatilas) too.
I think initially it was just a general term synonymous with “enlightened/realized person,” but then Buddhism and Jainism gave it specific definitions in their traditions.
It predated it. I. B. Horner devoted a whole chapter to this in her 1936 book, The Early Buddhist Theory of Man Perfected.