If you're mapping for a mod, cleanest way is to just use misc_model. For vanilla there are a couple of hacky ways.
One is to use a func_illusionary point entity and set its "model" to "progs/demon.mdl", and then "frame" to the desired frame ("53" for a dead fiend). However you'll need to precache the model by using another live fiend somewhere else, and you won't be able to rotate the model.
Another way is to add a regular fiend and set its "nextthink" to "99999999" (
see also). It will be frozen in its first frame and will contribute to the killcount, but won't be shootable.
Finally, you can just add a fiend and surround it with a func_wall made of skip.