After getting detention for wandering the halls after hours, you've been sentenced to fulfill your detention out in the Forbidden Forest with Hagrid.
While in the dark forest, you happened to see a creature and asked Hagrid what it was.
It's a shame though, since Hagrid may have had a bit of mulled mead before coming out tonight. All he can do is spout of nonsense.
It's your turn to figure it out on your own.
Hagrid says,
music
Cerberus
3 Heads!