The Calvin and Hobbes Wiki
Alien- Gurl

Gurls are an alien race in the Spaceman Spiff universe. A specimen was seen on an uncharted planet in a single Sunday strip.

The featured Gurl appeared as a very large, greenish-brown, tentacled being with a vertically stacked pair of eyes. Its body is stout with a large, antennae-laden head akin to a rotting prune. It does not appear hostile, as it did not try to act against Spiff even though he came provokingly close.

Although only a single specimen appeared, since the planet was uncharted and the Gurls were listed in the Saucer's on-board computer before Spiff came across one, Gurls must exist on other planets.

The creature's name is an obvious play on the word "girl", and their real-life counterpart was Susie Derkins.