While still very early on, mock up shots of a Dinosaur based combiner, loosely resembling the Dinobots (or at least their animal-modes), is in the works from TFC Toys.
Check out images of the proposed Combinosaur after the break. UPDATE
: More pictures have come to light of the individual dinos, all from ACToys. There are CAD designs for the T-Rex, Triceratops and Pterodactyl, plus finished renders of the T-Rex. Click here
for the original sources. 2005 Boards member ki1983ng informs me that these are indeed intended to be third party Dinobots who can combine to form The Beast
, but everything is currently still in the planning phase.