Enemies do respawn according to all current information. Certain enemies probably will not, such as the High Dragons.
Additionally, enemies have a Population that can be impacted by excessive killing. Most creatures can't go extinct in the game world, but wiping out the entire forest's population of wolves will result in no wolves for a while, and a slow recovery.
Furthermore, different animals, monsters and people will fight each other, likely based on internal Faction/Creature-Faction relations (Similar to how Elder Scrolls games do that).
So for example a "Nug" might be in a faction internally identified as "Nonhostile_prey_faction", which is hostile to nothing. But a wolf might be in a faction called "Hostile_Lesserpredator_Faction" which is hostile to the Nug's faction resulting in a simulation of a predatory behavior. This is an obtuse example drawn from a game that handles faction dynamics with the subtlety of a sledgehammer, but I think it'll serve the point.