Muggles' Guide to Harry Potter/Magic/Blood-Sucking Bugbear

Overview
A Blood-Sucking Bugbear is noted in passing in Harry Potter and the Chamber of Secrets. Hagrid mentions that his roosters are being killed by "either foxes, or a Blood-Suckin' Bugbear", and needs to ask the Headmaster for permission to put a charm around the henhouse.

Analysis
As a plot element, it is quite possible that this is only mentioned in order to point up the fact that roosters are being killed. The killing of the roosters is a clue to the nature of the occupant of the Chamber of Secrets. As story-telling, mentioning blood-sucking bugbears as well as foxes adds an element of grimness to the atmosphere of the story, and contributes to the reader's sense that the magical world and ordinary world intersect at Hogwarts.

Greater Picture
In fact, it is Ginny Weasley who is killing the roosters, under the orders of Tom Riddle, acting through his diary. This is because a Basilisk can be destroyed by the rooster's crow; if all the roosters have been killed, the Basilisk occupying the Chamber of Secrets will have a longer time when it can be allowed out to roam and hunt.

Again, it is this antipathy to roosters which is meant to give the reader an idea of what Hermione is looking for in the library, when it dawns on her what the monster in the Chamber must be.