Vermin.
I don't know if someone already mentioned him here, or let alone if he's considered "forgotten" enough to be mentioned here, as he still shows up every once in a while, but I shall still note that at the very least, Edward Whelan, has long been forgotten.
Edward Whelan, whom creator J.M. DeMatteis invested quite a bit on the first half of the nineties, confronting that he was once abused by his father, battling the animalistic Vermin, and winning, followed by contently becoming a productive member of staff in Ravencroft, loyal aide of Dr. Ashley Kafka, an actual successful rehabilitation from her.
Then, after not showing up for a while, subsequent Vermin appearances since, regressed back to sewer living cannabalistic rat man?