Doctor Dillamond is a character from Gregory Maguire's non-canonical novel, Wicked: The Life and Times of the Wicked Witch of the West.
He is talking goat who works as a Life Science teacher at Shiz University (the school where Elphaba and Galinda/Glinda attend). He was once widely revered for his knowledge and experience, but at the time of the novel, the Wizard has put forth a series of laws that restrict the rights of Animals, and these have greatly diminished the Doctor's influence. Doctor Dillamond is assisted by Elphaba and a Munchkin named Boq in researching and conducting some controversial experiments to discover the biological differences between humans, animals and sentient Animals, with the hope of proving that, since humans and Animals are the same, the Wizard's bans have no scientific basis. Eventually, another character, Ama Clutch finds him dead in his office, his throat having been slit. Ama Clutch later claims that the Goat was murdered by Grommetik, the mechanical servant of headmistressMadame Morrible. However, this statement is never conclusively proven. Morrible claims that Dillamond fell on a broken magnifying lens, and that Grommetik, finding him lying on the ground, had merely been checking for a pulse when Ama Clutch arrived. Elphaba remains certain that Dillamond was murdered, and continues his experiments later in life.
In the Musical
In the musical adaptation of Maguire's work, Doctor Dillamond teaches History as opposed to Life Science. He is the unfortunate victim of the the Wizard's attempts at controlling the Animal population - especially those that he fears might be teaching subversive or anti-Wizard material. Arrested and removed from his position at Shiz University, Dillamond is transformed into a regular, non-speaking, non-communicating goat. This confirms his earlier worries expressed in the song "Something Bad" in which he tells Elphaba about rumors of Animals losing their powers of speech. When Elphaba discovers Dillamond as a prisoner that can no longer speak, she resists the Wizard's offer to set the Ozians straight about her reputation, an enormous step in her transformation into the Wicked Witch of the West.
Glinda says during the show that she will continue Elphaba's work. This does imply that Doctor Dillamond could be turned back to normal.