The Karkinos is considered a large predator and therefore does not aggro other predators when ridden (with the exception of other karkinos, rock drakes, etc.), making it a great mount for relatively safe traveling!
HOWEVER!!! Smaller predators WILL attack if your karkinos is holding a prey creature in its claws, so keep this in mind while transporting tames and wild creatures!
If you're carrying a tamed Doedicurus in your claw the Doedic will automatically start harvesting any stones you hover him over. Just start pumping points into your Doedic's weight because it will stop at about half of its own carry weight. I've yet to discover another tame it will work with though...
Be careful when carrying a Dino that has a tribe mate mounted on it. I was walking up a hill carrying my anky with a tribe mate on it and they went under the map and died. RIP ANKY AND SHINEHORN. Tribemate was eh anyway.