Cave Crab

According to the game database, red with black shell is the rarest colour (1 in 26, or 3.8%) while blue with yellow shell is of medium rarity (5 in 26, or 19.2%). The remainder are purple with teal shell (20 in 26, or 76.9%).

These crabs spawn around the Blackwater Marina, in the north of Undermine. Most spawn points are around the southernmost of the 2 lakes, with only 1 spawn point at the northern one. I've only ever seen one up at a time and it looks like a new one is spawned a couple of minutes after the previous dies/is captured. It also looks like there's no point in killing the non-pet critter versions, as a new battle pet will spawn regardless of whether there are any free spawn points or not - in fact I've witnessed a critter being turned into a pet version on multiple occasions (in a different colour, so you can't strategically keep critters of a certain colour alive to force the colour of the pet spawn).