Notes:

A: He was in the prototype pilot before KND and is a regular character in KND*
B: Both of these showed up in the initial pilot "No P in the Ool"
C: Was the first promotional villain
D: The cannon timeline's first villain (That we are aware of)

Also, a secret E: Delightful Children From Down the Lane- as they were the first production series villain(s) in episode "Operation: C.A.K.E.D."

*Some would argue his "Villain" Status, but as he sides mostly with other villain adults, he is considered one, albeit a passive villain.