There are actually two varieties of hairless guinea pigs. The Skinny Pig, which does actually have a bit of hair, and the Baldwin guinea pig. While some people find their unique appearance unappealing, others find them quite irresistible.
There is some controversy about the introduction of these guinea pigs to the pet industry. Concerns about their immune system function and overall hardiness have been raised, though this appears to depend more upon their line and breeding rather than the fact that they are hairless.
Through careful breeding, it is thought to be possible to produce hairless guinea pigs that are hardier than their ancestors. While there are or have been some lab strains of hairless guinea pigs with reduced immune function, it seems that it shouldn't be assumed that hairless strains should be any less hardy than their hairy counterparts.
Their care is much like that of other guinea pigs. However, lacking a coat they are a bit more sensitive to temperature extremes and must be protected from drafts as well as direct sunlight. They also tend to eat more to maintain their metabolism and body heat (an excellent quality diet is a necessity, but should be provided to all guinea pigs, hairless or not).
Young Baldwin Guinea Pig
This young Baldwin has a bit of hair, which will be lost. They are born with a full coat of hair. After two to five days their hair begins to fall out, starting at the head and progressing to their hind ends. By the age of two months, they are completely bald.
The Baldwin guinea pig was discovered by Carol Miller, a breeder in California, the result of a spontaneous, recessive mutation in her White Crested guinea pigs. Because this is a recessive gene, if a Baldwin is bred with any other guinea pig, including hairless varieties other than Baldwins, the progeny will have hair. In order to be hairless, they must get the recessive gene resulting in the Baldwin type of hairlessness from each parent. If you breed two Baldwins, all of their babies will be Baldwins.
Adult Baldwin Guinea Pig
As an adult, the Baldwin guinea pig is completely hairless. As they developed from White Crested guinea pigs, they have wrinkles and folds on their shoulders and crowns in the areas where the crest would be. Their skin has a rubbery texture.
They should not be exposed to direct sunlight, and they don't tolerate cold temperatures, so they should be kept indoors. They should be provided with a small box to crawl into where their body heat will serve to keep them warm when they so desire.
The Skinny Pig was developed as a cross breed between haired guinea pigs and a hairless variety of Hartley lab guinea pigs. The Skinny Pig still has a bit of hair, especially on their nose, feet, and legs.
Skinny Pigs are born without much hair and stay that way, but they have different skin pigmentations. They have a variety of color patterns, including Dutch, Tortoiseshell, and Himalayan.
The hairlessness of Skinny Pigs is due to a recessive gene. If you breed two of them, their progeny will all be hairless. But if you breed with a haired variety, the progeny will be haired but carry one recessive gene.