Beefy, powerful, fast, good lifter, supports platforms, can be tamed on foot with much less efort and risk than Mosas (still not easy). For sure this thing is the most versatile mount after Mosas, Quetzals, Wyverns and Mantis (oh boi, you NEED a Mantis).
Forget the trap. If you're riding on an itchy, hit the one you want to tame with a tranq arrow. It'll aggro your itchy, but if your itchy has decent HP, it'll probably push through. Just make sure when your itchy's being attacked, lure the plesiosaur a little higher, near a rock if one's avaliable. When it's attacking your itchy, hit it with arrows. It'll torpor run, and keep up with it until it's unconsious. If I were you, tame it with minor soothing balm. It's cheap. I tamed a low lvl 2 male with 1 rex kibble and a minor balm. I had used the aggro itchy strategy, and knocked it unconsious and it tamed with one bite. Also, to the person who said Plesiosaurs are the Quetzals of the ocean, I will also change your mind. As everybody knows, itchy's are super fast sprinting. Well when I tamed my Plesiosaur, I decided instead of requesting, i'd bring it home. Well, it kept up with my itchy, even though the itchy was sprinting. Also, you cannot request Plesiosaurs in shallow water, only in deep water. They're pretty stubborn. Please ^ if this was helpful!