So i almost cleared out a small dungeon last night beneath an old ruin. I was unable to reach a room, because a large cage blocking the door, but managed to kill with my bow most of the battlemages and rogues via shooting through the cage. I was impressed you could do that with this old game.

Anyhow, i later returned and the dungeon and specifically the room i mentioned above was full of zombies. There were no zombies in my last visit. No other enemy types for that matter.

My first thought was since the dungeon was almost cleared of enemies the game randomly choose a new type of enemy to fill the dungeon with. But then i thought, what if the dead battlemages and rogues i killed turned into zombies? is this a thing? or just random chance i got zombies upon returning?

Also in general do dungeons in DF usually respawn new enemies if you clear them?

I think each dungeon has a particular class/classes of enemies that can spawn within it, and the game can kind of randomize within that/those. I don't know all that much about how that works, though, so I'll let someone else answer that. (Just like later Bethesda games, though, Daggerfall does choose enemies it considers appropriate to you current character level, so that can drastically change the threats you'll find in a given dungeon over time.)

With regard to enemies respawning: the original game (and AFAIK the unmodded Unity interpretation) doesn't remember anything about the state of any dungeon that you're not currently in. This means that the instant you exit, any exploration progress (i.e., mapping out the dungeon) is wiped, and any loot you might have dropped or left behind is gone forever. It also means that if you go back into a dungeon you've previously visited -- whether in-game months ago, or less than a minute ago -- the dungeon will generate new enemies and treasure appropriate to its type and your level, irrespective of anything you might've done in that dungeon previously. (If there's still an active quest target within, it will also re-randomize its location from among a small pool of possible pre-set locations.)

I'm not sure if any of this has been changed by mods included in Daggerfall Unity - GOG Cut, however.

There is a DFU mod "Persistent Dungeons" which "remembers" the state of a dungeon when you leave it. I haven't used it, nor can I give you more details.
It's described and discussed (including a download link) in a DFU forum thread - search for "daggerfall persistent dungeons".

Yes and no. Daggerfall is heavily level scaled, so enemy spawns get changed as you level up. However, all dungeon has a dungeon type which dictates what kind of enemy spawns there at each level.

See the table with dungeon types and monsters here:

[url=https://en.uesp.net/w/index.php?title=Daggerfall:Dungeons]https://en.uesp.net/w/index.php?title=Daggerfall:Dungeons[/url]