Well having in mind of the Owlite shield Looks like an owl, and the fact that they're called OWLites we could assume that they were some kind of owl-ish animal/monster/thing. (Same with the kats)
the zombies on the other hand have a mouth with teeth instead of a beak, so that kind of rules out the possibility of them being dead owlites.
The feather drops are just there to craft the magic cloak armors, since you know... owlites... wisdom... magic.
They are called "Dust" Zombies, and it is more likely that they are Almirian citizens rather than owlites. As far as zombies and dreadnaughts go they seem to have more linking them to Almire than the Owlites, One prestige mission has you enter an almirian graveyard to retrieve a Codex (where you find Almirian crusaders, shadow guards, and zombies), another you must travel through a graveyard to meet the spirit of an almirian crusarder, lastly is the Firestorm citadel (where Slags and royal guards are). Certain zombies even have a blackened crest shield that they carry. The fact that Kats show up in some of these levels proves that the Almirians and the Kat tribes existed together (most likely at war considering the current info on both).