These guys will never make a move to attack you, in fact, they are more likely to run away, and are rare due to being a snack for kapros and sacros. You can pick one up with a tame kapro, so you can actually save them.
If your struggling a to find one check the nearby sarcos, kapros and snakes in the swamp to see if they are chasing something. Chances are they’re chasing a diplocaulus. Or a phiomia. Those darn phiomias are always getting chased…
I happened to go prime meat shopping and scoop him up. Never seen one before. Loved it. Found a mate! Made them mate! Egg sit it in the water... baby Explode. Lmaoooooo never have I ever seen a Dino explode before. Was so piss off... I laughed...went to bed. Lol
If having trouble finding one in the swamps try bringing a parasaur with you and use its detection ability to see all nearby carnivores hiding in the water and walk up to them to check if they are a diplo. as long as you have some sarcos or barrys to clear out the piranhas you will find one in a couple of minutes easy.