Boomer is Seth's pet boa constrictor. He lives in the train with Seth and appears to have the run of the place when Seth is at home.

Boomer must be kept in his terrarium when he's home alone, otherwise he could find a way to get outside. (This isn't a big problem in the warmer months, but cold temperatures can kill a snake.)

While we're not entirely sure where the idea for Seth's pet comes from, Melissa has said in her blog that she had a few exes who have had pet snakes, so we can asume it originated from that.