Croaker was a person who worked in the Ministry of Magic. He was an Unspeakable who worked in the Department of Mysteries. He attended the 1994 Quidditch World Cup. Arthur Weasley was familiar with him, but not with his work.
A "Croaker" is someone who habitually predicts evil; it has that sense because croaking is a noise made by crows and ravens, who are considered death omens, as they are carrion eaters.
- Harry Potter and the Goblet of Fire (First mentioned)