Carnos are excellent mounts and hunting and battle companions. At mid-level, my pack of five have speed slightly superior to a raptor and strength comparable to a low level Rex. Focus on building melee, speed and stamina. Combine them with a couple raptors, a Rex and a Trike and there will be very little you can't overcome.
Tamed carnos are excellent almost all around: their health, attack speed, and base damage allow you to take on most of the creatures on the island, including Rexes and Alpha Raptors. Having a mated pair greatly strengthens this quality. They run fairly fast and can carry a lot, making great transport mounts. They are a fairly good harvester of meat and prime meat.
Their only real disadvantage is a slow stamina regen, even when stood completely still.