Slowpoke
Due to its marine nature, shape and association with shellfish, Slowpoke may be based on various species of otter. It may also be based on a giant salamander or hippopotamus.
Slowbro
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.
Slowking
Slowking's shell makes it similar to hermit crabs, though it is physically similar to giant otters, giant salamanders, and hippopotamuses. Its shell "crown" 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. It is also based on a king, as the growth around its neck is based on a king's ruff while its shell is similar to a crown.
-Bulbapedia
Hope I helped.
