What is the only animal born with horns on its forehead in both genders?

The only animal born with horns on its forehead in both genders is the giraffe.

While both male and female giraffes have horns, they are more pronounced and larger in males. These horns are actually ossicones, which are bony projections covered in skin and hair. They are not true horns, which are made of keratin like human fingernails.