It's a little hard to tell from your question whether you're talking about a mole or a generalized discoloration. So it is important that you see your doctor
. If what you're talking about is clearly demarcated, it can be any number of skin lesions, including a mole (which could be congenital), it could be one of several different forms of skin cancer (squamous cell, melanoma), or it could be blood vessels beneath the skin that are causing a generalized discoloration (this could be benign and also be from birth or could be a malignant growth). One definite possibility from your description is a fungal infection, which can occur in the groin due to the warmth and moisture, and can cause changes in skin color and even hair growth. There can also be contact rashes or rashes due to irritation or chafing. Sometimes the skin on the inner thighs can be discolored, which can be normal or can represent early diabetes. Given this wide range of possibilities, I would strongly advise you to see your doctor and have them take a look; the treatment depends on what the actual diagnosis is and it's important to have someone assess your skin and make sure that nothing serious is going on.