Making new enemies?

Hey, I've had a little bit of experience with making games but I'm completely new to Doom modding, so I was wondering if someone could help me here. Making levels and placing objects and things seem simple enough, but is it possible to make new enemies or edit the AI? For example, make a new version of the imp with different graphics, that doesn't shoot fireballs, or make a version of the lost souls that has to move along the ground? If possible, in addition to the original version?

If this is possible, could someone give me some idea how to do it? Thanks.

Yes a lot of what you say is possible - I'm just not that experienced with it but I've seen it done before, especially with the newer programs that are out. Back in the day I used to mess around with Dehacked, which is an older, more restricted sprite editor, and has tags for things like whether or not gravity affects it. I recall making a "grenade launcher" of sorts by making rockets affected by gravity and increasing their damage/radius..

Anyway I'm sure there are others here who know much more about such things than I do, and I'm certain there are programs or methods of modifying probably any aspect of a monster whatsoever. As for AI, they made some pretty decent/deadly bots many years ago that were almost as good as humans players, and that was around a decade ago.. And there are tons of new maps and whatnot that have all kinds of new monsters and other sprites in addition to the originals (Knee-Deep in Zdoom for instance). So I know it's possible, and the results are out there, but I'm just not an expert with that, though there should be some here who are.

Which port do you intend to mod for?
Making new enemies for Doom itself is difficult (you have to use DeHackEd and it's quite limited). Advanced ports such as ZDoom, Vavoom or EDGE have their own ways of adding new elements to the game.

If you're planning to use ZDoom, you're in luck because the ZDoom wiki is very complete and you can read how all the Doom, Heretic, Hexen and Strife actors are defined, so it's a perfect source of example to learn how DECORATE scripting work and how you can modify existing actors or create brand new ones.