IIRC, it's some post-surgery support thing, adopted or appropriated by troons as a sort of mascot. The most notable example that I can remember is some guy who boasted about his bottom surgery, showed his little shark plushie in picture of himself during recovery, and then proceeded to lose his fucking mind when he finally clued in that his neo-twat was just shallow, nasty meat pocket full of never-ending crud and shitstink.