Dreugh are for all intents and purposes beasts, even if there is lore about the Dreugh supposedly ruling a previous/adjacent version of Tamriel or intelligent Dreugh living at the bottom of the sea, but even those source don't deny that they are beasts now (or just when they go on land for mating).
Accordingly, they should count towards your endeavor to kill beasts.
Thank you for coming to my ted talk.
(I was in Stonefalls, at Vivec's Antlers trying to kill Dreugh for the Endeavor but didn't get any progress on it.)
Edit4: Nix-hounds, Mudcrabs, Netches, Guars, Spiders don't count either in Stonefalls.
I have now moved to Deshaan and there Wasps, Mudcrabs, Giant Snakes and Kagouti don't count. Bog Rats in Deshaan do count! I will expand this list as I come across more cases.
Edit 5: Across multiple zones only mammals count as "beasts", however trolls don't count as beasts either and they have fur!
Edit 6: Salamanders do not count towards it either, even though they drop Foul Hide. I was sure the Foul Hide was the actual condition behind it, but apparently not. This is a complete mess!
Edited by Ratzkifal on July 13, 2021 2:44PM This Bosmer was tortured to death. There is nothing left to be done.