Terrible Teach was supposedly a human man.
According to a phony treasure quest organized by Yellow Beak and another retired sailor in the early 1960's, Terrible Teach was a notorious pirate who buried some treasure in a cave in a seaside port where Woody Woodpecker once operated a restaurant. Terrible Teach's ghost later supposedly appeared to his descendant (Yellow Beak's friend, playing the part of a nemesis) to tell him where the treasure was. At the conclusion of the role-played treasure hunt, Woody, Knothead and Splinter came across what seemed to be the ghost in question, though he was actually Yellow Beak's friend in disguise.
Behind the scenes
Terrible Teach is mentioned throughout the 1963 story Woody, Knothead and Splinter Search For Pirate Treasure, which notably does not explicitly state whether Terrible Teach ever really existed. The name of “Teach” for a pirate was likely inspired by Edward Teach, the real name of Blackbeard both in real life and in the Prime Universe, though they do not appear to be intended to be the same individual.