Its tail has a Shellder firmly attached with a bite. As a result, the tail can't be used for fishing anymore. This forces it to reluctantly swim and catch prey.
(From Bulbapedia) SSlowbro is a bipedal, pink Pokémon with a tan, striped belly and a tan muzzle. It has large, vacant eyes, curled ears, and two pointed teeth protruding from its upper jaw. On its hands, it has three clawed fingers, and two clawed toes on its feet. There is a Shellder attached to its tail. This Shellder has a gray, spiraled shell with darker gray spots. There are several spikes across the surface of the Shellder, and it holds onto Slowbro with many sharp teeth. If its tail or the Shellder attached to it is broken off, Slowbro will revert to its unevolved form Slowpoke.
When Slowbro Mega Evolves, it becomes engulfed by the Shellder to the point where only its head, arms and tail are visible. The spiral Shellder now has an opening at its tip, allowing the tail of Slowbro to stick out, which it balances on due to its legs being engulfed. It can move by bouncing, using its tail as a spring. The spikes on the Shellder are now reduced to four on the bottom section of its body, with the dark spots only visible on its middle section.
Slowbro has lost the capability to feel pain due to the Shellder's poison. However, it apparently becomes inspired when the Shellder bites down on its tail. While it is capable of unleashing powerful psychic attacks, Slowbro is typically rather peaceful and usually prefers not to fight. Because it can no longer fish using its tail, it instead swims to catch its prey and allows the Shellder to feed on its leftover scraps. Slowbro tends to live near ponds, lakes, and seashores. Mega Slowbro's energy has gathered into Shellder, which has increased its hardness. This allows it to act as an armor so hard that nothing can scratch it. Slowpoke is virtually unchanged though.
(From Bulbapedia)Slowbro's shell and lifestyle are somewhat similar to hermit crabs, though it is physically similar to giant otters, giant salamanders, and hippopotamuses. Slowbro's appearance may also be a reference to the mythical sazae-oni, a magical sea demon of Japanese folklore distinguished by its spiny "sazae" or "turban shaped" snail shell.
Its parasitic relationship with Shellder may be inspired by leeches. Interestingly enough, Slowbro is known as the Hermit Crab Pokémon and many real-world parasites, including leeches, often target crabs.
(From Bulbapedia)Slowbro is a combination of slow and bro (short for brother). It may just be used in a general sense, or it could refer to the brotherly/symbiotic relationship shared by the combined form of Slowpoke and Shellder.