I'm not good at designing races or monsters, so I'm going to put up what I'm trying to make and get advice on how to stat it out.

Fluffish:
The Vomada are a proud race of reptilian hunters who can swim through the earth. Though slow on the surface, underground they can outrace a human. They specialize in hit-and-run tactics, aided by their ability to spring from the ground and attack before diving under the surface again.

Crunchish:
I'm looking for a burrow speed of 40 or 50 ft but walk speed of 20ft or less, with the equivalent of Fly-By Attack as a possible bonus feat. Ideally these could be weak enough to be used as PCs, but I'm not sure whether a monster that that appears, attacks, and then vanishes from its enemies' sight every round can be balanced for low levels. Perhaps a low starting burrow speed and a speed bonus based on HD?

This picture gives a decent approximation of what I'm aiming for. My idea for this started there. My original description of the race for an evoltuion game is here.