I have seen several dogs (and had several head of cattle) that have been bitten by rattlesnakes. The cattle are big enough that it just makes them sick for a while, but a big knot will form and drain for several days. Dogs aren't always so lucky. If they take a hit on the head, they will usually get sick and swell, but will live over it. If they get bitten on the foot, they don't live very long. Snakebites are usually indicated by some massive swelling and a large area of tissue around the bite would rot off.