It is revealed in South Park: The Fractured But Whole that Craig has had 4 guinea pigs named Stripe; the Stripe he has now (Stripe #4) was given to him by his boyfriend, Tweek, presumably because the other ones either died or ran away.
Stripe #1 died when Craig's mom accidentally stepped on him, and broke his back.
Craig suspects his dad of poisoning Stripe #3 after he pooped in the dining room. It is also revealed that Stripe #3 died very recently, due to the fact that Craig still has (#3)'s toys and food dish tucked away in his closet.
Stripe also plays a larger role in South Park: The Fractured But Whole as a part of one of Super Craig's Ultimate attacks. He also exists as one of the items that needed to be retrieved before you can get Craig's laptop.
Stripe's appearance frequently changes from the traditional construction paper art style to live-action. In the traditional style his torso and limbs are covered in dark brown fur, and the fur on his head is white with light brown splodges. He also has a little pink nose.
In his live-action appearance his fur is primarily colored light brown, with a few white and ginger splodges here and there.
Craig sometimes dresses him up in a pirate costume.