Rats are common in Sam & Max's adventures.
In most comics, the rats are background characters in scenes, seen stealing food out of trash cans or running across a panel. However, the moon rats in Bad Day on the Moon are the size of Sam and are important to the plot.
Steve Purcell had pet rats, as mentioned in Telltale's article History of Sam & Max part 1. One was named Ernie, who inspired the rats in Steve's other comics. Toybox, another comic by Steve, has a rat named Ernie, likely named after his own pet rat.