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.