I can think of a couple reasons some one might name a hound Rattler.

1. The sound the chain makes as the dog shifts around on the end of it where he is chained on the porch or under the front steps. Paw, sittin' on the sofa cleaning his gun, "That pup out there sure is a live wire. Just listen to him fidget." Maw, threading the last of the seasons bean pods onto strings to dry into leatherbritches for the winter larder, "Yer right, Paw. He orta be called Rattler, the noise he makes with that chain."

2. When you watch a dog face off with some small animal that fights back, say, a groundhog, the dog will kinda lean back, draw back it's neck while keeping the angle of the nose down, then leap forward and strike, grabbing the groundhog by the throat. The motion does rather look like that of a snake, poising, then striking.

3. None of the above.