To earn this Trophy create a Skaven team in a Solo or NAF Online League. Once one of your Skaven earns 6 Star Player Points (SPP) you get a Skill-up roll. If this roll comes up doubles you will be able to select a Mutation. There is roughly a 17% chance (1 in 6) of rolling doubles so with a little luck you'll roll doubles quickly and be able to pick the Horns Mutation needed for this Trophy.
This can also be combined this with the Unnatural Trophy, which only requires that you pick a Mutation as a Skill-up.