There is only one known poisonous snake, the Asian Tiger Snake. It is not poisonous naturally, but due to the toxicity of the frogs and toads it eats it absorbs their toxins in its nuchal glands and then secrets them back out through the skin around it's neck.
This sequestering of poison while not common is not unheard of in the animal kingdom. For example birds in New Guinea are believed to sequester poison from insects they eat.
Poison and venom are not the same thing. Venom is injected subcutaneously via a bite or sting, while a poisonous animal or plant releases the toxins through its skin, or when ingested.