What animal has been thought not to be affected by snake venom?

Pigs are thought to be immune to snake venom as they have rarely been seen to be affected after being bitten by snakes. But this is actually not true. Recently it was thought that the pig's thick skin means that the snake bite simply does not penetrate it. It has also been thought that the thick layer of subcutaneous fat which, due to being poorly supplied with blood vessels, helps to prevent the spread of poison. But, as most venoms pass easily through vascular layers of fat, this hypothesis also seems unlikely, and is probably due more to the sheer mass of the pig. Scientific tests done on pigs have shown that pigs are indeed susceptible to snake venom.

So, although the pig is thought to be immune, this is in fact not true.