A net
The manta ray looked normal for the most part, but when it swam closer to Peter, it flipped his fin over and you could see that it had a net attached to it. The net looked heavy and was weighing the poor creature down. Peter felt like something needed to be done here.
This manta ray could not go through the rest of its life with a net tied around its body. That wasn’t going to be good for its physical state over a long period of time. Not to mention that a lot of animals get tangled up and can even choke in some cases.