You're talking about very different levels of classification here! Cats and dogs are two of the 12 families (Felidae and Canidae, respectively) in the Order Carnivora of the Class Mammalia in the Phylum Chordata. Family names end with 'idae' and the adjective form ends with 'ine.' Technically, they are not felines and canines, they are feline and canine but felids and canids. Canines are teeth.
Butterflies come from five families in the Order Lepidoptera (which also includes moths) of the Class Insecta in the Phylum Arthropoda. Thus, butterflies are Lepidopterans, but not all "Leps" are butterflies.
Snakes represent 12 families that comprise the Suborder Serpentes of the Order Squamata (which also includes lizards) in the Class Reptilia of the Phylum Chordata. So snakes are serpents, which you probably already knew.