My daughter(1yr)is having a dark mark with thick hair on it, it s about a lemon sized, on her arm, it was there on her birth itself.its really dark.

Hairy nevus?: It sounds as if your daughter has a type of birthmark called a 'hairy nevus'. They can be a cosmetic concern, but also may develop malignancy. Nevi (moles) present at birth (congenital), >1cm, multi-colored, irregularly shaped, irregular borders have an increased incidence of malignancy. This one is >1cm and congenital. Have her see dermatologist to determine if/when it should be removed.

Congenital nevis: Congenital melanocytic nevi are considered developmental errors in the pigment forming cells of the skin. There are conflicting opinions on management but most recommend simple yearly monitoring by the doc.There is rare risk of a skin cancer arising from the area & its odd coloration makes noticing the change harder. Have your doc or a dermatologist confirm the dx.

Sounds like a Hairy : Nevus, a type of birthmark, a mole with hair growing from its surface. Because there is a possibility that it can become cancerous, it's wise to have her pediatrician follow its growth & refer to a dermatologist or plastic surgeon when it's time to remove it surgically.