It's been over 35 years since I read the book but I came away thinking it was a one-time-only thing and not a cryptid by any means. That silly statue they erected in Point Pleasant, W.V. is an abomination.
Keel himself stated he was not a UFO-logist or a cryptologist but a demonologist!
On the other hand, it could've been a large specimen of a known species of owl.
Yes, I would figure it was demonic.