"Snotrod" is a stereotypical muscle car who's friends with the three stereotypical "ricers" that come upon the main characters on the interstate. It appears that "Snotrod" is an early 70's 'Cuda or Challenger. It has a black stripe on it that looks kind of Dodge to me, but I'm not sure. It's definately a Chrysler product.
There are a number of running jokes with "snotrod". Because he's a muscle car, he leans and swerves all over the road, while the ricers just zip around. He has a big ol' bug-catcher and blower sticking through the hood, and this must be his "nose" that makes him sneeze. Every time he sneezes, he seems to get a shot of nitrous.
If you stay for the credits, during the credits the 4 cars including snotrod get caught speeding and have to pave more of the road.