Dimetrodon was a "pelycosaur" a basal synapsid, the sister group of the sauropsids, it was a large sail backed predator which lived in the late permian this is a picture of one:
Some sites of interest:
http://www.talkorigins.org talk.origins archive
To contact the Author:
mailto:brett@dimetrodon.demon.co.uk
For a few patches for roguelike games, see here.
Vlet's Mod of Hotline 1.9