Once the team figures out which of the many creatures available it wishes to include in the game, what comes next is a matter of sketching out the possible designs and sculpting details in basic 3-D models. It’s a fairly involved process, the sort you can see in depth on the official site if you want to know how a monster goes from a twinkle in the design team’s eye to that irritating snarling thing you hate fighting.
Neverwinter has an advantage in terms of enemy design, since years of Dungeons & Dragons have given it huge amounts of material to work with for new monsters, even if you disregard options like the xorn. That doesn’t mean it’s exactly easy to design new critters for the game, though, just that the hard work shifts away from figuring out what possible new creatures might inhabit the game. A new development blog is about just that, not coincidentally.
Source: Creating Creature Art