Barnabas the Barmy was a wizard who famously tried to teach ballet to trolls but was clubbed for his efforts.
Barnabas is depicted on a moving tapestry on seventh floor of Hogwarts which shows his foolish attempt to train trolls for the ballet. The tapestry hangs opposite the entrance to the Room of Requirement (OP18, OP26, HBP20).
The word "barmy" means crazy or mad, according to the Oxford Dictionary.
In 2013, the Warner Bros. Studio Tour London organized a Troll week, which included “Trolls in Tutus”. These evening performances featured Barnabas the Barmy’s attempt to teach ballet to trolls. There is a behind-the-scenes video of the performance available online.