Two snakes appear in the game in terrariums at the Vanilla Unicorn and Yellow Jack Inn. However, they do not move and exist only as static "objects". A live snake appears in a commercial for the Grand Senora Desert.
It is possible that they existed in the beta version as active models like in Red Dead Redemption, but they were later cut in development for the final version. Snakes also appear in the film Capolavoro, along with Horses, suggesting that snakes were developed for the game (or to be precise, reused from RDR).