Short haired guinea pigs do not have to have their fur cut. It will simply grow to its natural, short length. Long-haired breeds, however, require haircuts to make them more comfortable. If their hair grows too long it will become knotted and matted, and hay, droppings and food can get stuck in it. The guinea pig will be unable to remove these items themselves.
A Guinea Pig in need of a trim
If you want to give your guinea pig a hair cut, be careful not to cut too close to the skin, and be extremely careful not to snip its skin or ears. The haircut should not be carried out by children or anyone nervous about the procedure. You can always get a professional to do the job for you - in the first instance, at least.