Morrigan was a famous Irish witch whose Animagus was a crow (Pm).
Morrigan is a direct ancestor of Isolt Sayre, the founder of Ilvermorny, who on the other side of her family is also a direct descendant of Salazar Slytherin (Pm). Isolt is described as 'the offspring of two pure-blood wizarding families', suggesting that Morrigan must have been pure-blood.
Other canon notes and references
Morrigan's descendent, Isolt Sayre, the founder of Ilvermorny School in Massachusetts, was nicknamed 'Morrigan' by her father, because of her affinity with nature (Pm).
In Irish mythology, the Morrigan (meaning "great queen" or "phantom queen") was a goddess of fate, conflict and fertility. She often appeared in the form of a crow or raven, who flew over the battlefield to instill either fear or courage in the warriors, as well as foretelling the death of individuals among them. (Wikipedia)