Anglerfish
Given the reputation of being the ugliest animal on the planet, I had to look up more details on the Anglerfish. Here’s a picture and some details about this deep-sea carnivorous fish.
Nevertheless, what caught my interest, apart from their fantastic and gory looking appearance is the following fact:
The male, which is significantly smaller than the female, has no need for such an adaptation. In lieu of continually seeking the vast abyss for a female, it has evolved into a permanent parasitic mate. When a young, free-swimming male angler encounters a female, he latches onto her with his sharp teeth. Over time, the male physically fuses with the female, connecting to her skin and bloodstream and losing his eyes and all his internal organs except the testes. A female will carry six or more males on her body.