Was browsing through the forums, and I have to say, its a real shame that this thread died. Let me just go ahead and.. necro it a bit.
With the release of Aberration, let me share my thoughts on those dinos a wee bit.
Roll Rat - PvE, A - Akin to the beaver, gathers wood, but does not have a smithy saddle. Its slow when its not rolling, and rolling consumes metal ingots as you do it.
PvP, B - Its rolling allows for quick getaways, and it can carry 3 players to hit a base real quick, and get out. Riders are protected from outside damage while rolling as well.
Ravager - PvE, S