No sources, but I have seen some documentaries on animals seeking out fermented berries for their recreational properties. This isnt as prolific as say, someone that uses cannabis every day, but there does appear to be "recreational drug use" in the animal kingdom.
If you consider catnip to be a drug then cats using catnip would qualify. Kittens don't get much off of catnip and a good percentage of cats in Australia are immune to the effects of good nip, however most cats enjoy it. It drives them crazy in sort of a sexual way. Catmint is a similar plant but far less powerful.