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!
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).
How to get high quality plesi platform saddles on mobile:
Head into the yeti cave (tame one of the wolves in the cave to do this). There is a big room with a bunch of snow covered platforms. One of the platforms has a red cave drop, and that drop’s loot includes Plesi platform saddles. I ran the cave ~100 times and got 2 ascendent plesi platform saddles and 2 other plesi platform saddle blueprints but I don’t even have a plesi yet lol.