Resheph

Dad always had a theory about Croatoan. He thought it was a demon's name, sometimes known as Deva, sometimes Resheph. A demon of plague and pestilence. – Sam, 2.09 Croatoan

According to Wikipedia,

"Resheph was a Semitic god of plague and war. He bore the head of a gazelle on his forehead and was an important member of the pantheon of Ugarit though not mentioned in Ugaritic mythological texts."

Christopher Siren identifies Resheph as a Mesopotamian God of pestilence (source).

This is not the first time Mesopotamianism has cropped up in speculation around Supernatural's mytharc: there's a theory that Dean's Amulet is a Mesopotamian figure.