Snivy is in fact based off of multiple species of snakes, according to Bulbapedia (below the Trivia section, it has an Origin section).
Snivy may be based on a variety of green vine snake species with a pointed snout, including Oxybelis fulgidus, Ahaetulla nasuta, and Ahaetulla prasina. Snivy's color scheme is similar to its species' possible namesake, the smooth green snake. The Pokémon's combination of snake- and lizard-like features is also similar to that of the Florida sand skink, a lizard that resembles a snake with small, stubby legs. The shape of its head, body, and the protrusions on its shoulders may be based on the bird of paradise or false bird of paradise flowers. Ken Sugimori has said that Snivy, Servine, and Serperior are based on French royalty, specifically mentioning the anime The Rose of Versailles as inspiration.
Oh, and, according to Google, oxybelis fulgidus is known as the green vine snake, ahaetulla nasuta is known as the common vine snake, and ahaetulla prasina is known as the Asian vine snake.
Oops, forgot Servine and Serperior. Servine has basically the exact same text for its origin as Snivy, but this is Serperior's origin:
Serperior is based on multiple species of snakes and limbless lizards, such as Oxybelis fulgidus, Ahaetulla nasuta, and Ahaetulla prasina (species of vine snakes), as well as the emerald tree boa, ball python, and possibly Burton's legless lizard. The shape of its head and the extensions on its collar may be based on the bird of paradise or false bird of paradise flowers, a cobra's hood, or a snake's shed skin. Serperior may also originate from Quetzalcoatl, a green, feathered serpent deity. Its body markings resemble the acanthus leaf motifs popularized during the Renaissance.
Ken Sugimori has said that Snivy, Servine, and Serperior are based on French royalty, specifically mentioning the anime The Rose of Versailles as inspiration. The fact that Serperior amplifies sunlight in its body, as stated in its Pokédex entry, may be an allusion to King Louis XIV of France, who famously referred to himself as the "Sun King". Its white face also alludes to the skin whitening fashion, mostly used by the European aristocracy in the 17th century. The dark green design on its chest seems to form a fleur-de-lis, a French insignia that is used on many European noble families' and nations' coat of arms. Along with its regal look, Serperior's ability to seize opponents with its piercing stare is a reference to the basilisk, the king of serpents in European legend.
Hope this helps! :)