Its four horns are a high-performance radar system. It uses them to sense sounds and smells, rather than using ears or a nose.
(From Bulbapedia) Sliggoo is a gooey purple Pokémon with the body structure of a snail. Its underbelly is a deeper shade of purple than the top part of its body. It is over twice as tall as its preevolved form, but remains relatively small. Its eyes are now green and large, with one eyelash on each eye; but since its eyes are undeveloped, Sliggoo is blind. It has two pale green spots on either side of its head. It has two small, underdeveloped arms, two sets of antennae atop its head, and a gooey snail-like shell on its back that looks like it is part of its gelatinous body.
It can excrete sticky mucous that can dissolve anything, coating its entire body with it. Because its eyes have devolved from a Goomy's, it has trouble discerning friends and food, often attempting to calmly melt and eat those it gets along with without realizing it. Instead, it relies on its four horns to sense sounds and smells. The protrusion found on its back contains its brain and other organs. It crawls sluggishly.
(From Bulbapedia) Sliggoo seems based on soft bodied gastropods such as slugs, nudibranch, and semi-slugs. Its snail origins as well as its Dragon typing and elongated body shape may mean it could also be based on the Lou Carcolh, a mythical serpent/mollusk creature from French folklore. It may also be based upon the concept of "living slime" in popular culture.
(From Bulbapedia) Sliggoo may be a combination of slick, slither, slippery, slimy, or slug, and goo.