These are some creatures I worked on which were featured in the Guild Wars 2 Manifesto video. We usually get a concept from the concept art team, but many times it's more related to the "feeling of the thing" than a specific outline or orthographic drawings. So, a fair amount of design work is executed by the creature artists. Things we try to consider are the silhouette, overall visual impact and readability from a distance, what environments the creature is likely to be spawned in etc. From a technical standpoint, we fit the models on to existing rigs (usually). So the one thing we can't adjust are the proportions (which drives me a little nuts).