"You can't hurt a baby!"
-- Hermione as Harry raises his wand to attack the Death Eater (OP35)
The Death Eater with the baby head is a male Death Eater whose head transformed back and forth from baby form to adult after falling into the Time Room Bell Jar during the Battle of the Department of Mysteries (OP35).
It is not certain which of the Death Eaters who chased Harry, Hermione and Neville into the Time Room during the battle was the one whose head fell into the Bell Jar (OP35).
An excellent analysis on the Harry Potter Wiki uses process of elimination to suggest that the baby-head Death Eater was either Rabastan LeStrange or Crabbe Sr. The discussion ends with this conclusion:
"I think it was Crabbe Sr. Rabastan is said to be a thin man while the baby-headed Death Eater's arms are said to be thick. Which would fit if he looks anything like his son. Also, Harry saw Rabastan in the Pensieve alongside Rodolphus, Bellatrix, and Barty Crouch Jr. and probably would have recognized him once his mask was off (as he does Dolohov, whom the book refers to by name once his mask is off)."
-- Ghostkaiba297 08:32, August 28, 2011 (UTC)