It could have been any generic demon that was summoned from that rock thing.
They didn't really use much of anything from the fairy tale except for that he wanted the baby and straw was referenced.
They barely even used the most famous guessing of his name part, just as an aside while he was chasing her through the house at the beginning.
Nothing much authentic in this. The characters from German folklore, but for some reason they had him speaking 6th century English dialogue like a nobleman... this wasn't supposed to be King arthur. They did reference his name as an incantation at the end but the fairytale was bastardised from the beginning
Another criticism is how badly this has dated, it was dated when released. I thought it was produced in the mid 80's. The attitude, clothes hairstyles and music...maybe it was supposed to pay homage to that period of horror or something