Ravan was half-brahmin and half-demon.His father,the great rishi Vishrava belonged to the Pulastya clan and mother Kaikasi originated from the asura clan.Since he was a brahmin and was the greatest devotee of Mahadev,it might be true that he was a vegetarian. However it might also be possible that he was a meat eater,due to the fact that he possessed demon genes.
Ravana was not a vegetarian.
As per UTTARA KANDA of Valmiki Ramayana, this is what Ravana did in king Marutta's Maheshvara yajna:
And then devouring the Maharshis that were present at the place of sacrifice, Ravana, satiated with their blood, again went to the earth. (English translation source)
The complete story of king Marutta's Maheshvara yajna is given here.