If you would like to tame a Diplocaulus, i would suggest to pick it up with an argentavis and drop it off at your base. Keep hitting it with tranq arrows and you should eventually knock it out. Keep meat with you to start the taming process
They are skittish but not anywhere near faster than humans. They have low torpor and health, so tranq darts are recommended. Tamed like any carnivore, kibble is Archeopteryx. Can be found anywhere in and around the swamps.