If trolls are supposed to have different horns from different signs, then how come Trizza has the same horns as Feferi, Meenah and Condy? Trizza isn't True Pisces.

The Peixes horns are pretty generic looking, no reason why they couldn’t have the same horn shape. Besides- Horns aren’t like symbols. Symbols are designs Trolls made to represent them, Horns are part of their biology. If it’s in the gene pool, shapes can just happen.