Well. I think Griever is a Great Lion. I think the wings can describe his bad side (XD well the only time you face him is when you go with Ultimecia). Have you ever noted that Squall said sometime that the one in his ring & necklace was a Lion? Named Griever of course. So I still think Griever is a Lion, not a Chimera or anything like that but a Lion. Greetings! XD
(Excuse my bad english I'm mexican XD)