The Rainbow Cat is a pet obtained from the Portal Pioneer Bonus Box.
If you own the Portal Pioneer Bonus Box, you can get an unlimited number of these cats (since you can get the bonus box over and over again).
The cat's rainbow fur glows, but does not give off light. The cat's eyes are the only part of its body that don't glow. As such, when the Rainbow Cat is in pitch darkness, its fur is visible but the eyes are not, making it seem as if it has black holes in its head.
It might be possible for players who missed the Portal Pioneer Bonus Box to still get the cat, by trading with players who have the cat in a Multiplayer game.