Those are the Padjal. They're Hyur who are chosen by the Elementals to be their mediators with the Gridanians. They're usually chosen as children, whereby they grow horns and stop
visibly aging (they don't seem to be biologically immortal). A few families are known for producing them more often than others, such as the Senna family - the three Senna children currently lead Gridania, with Kan-E-Senna in the highest position and also leading the Order of the Twin Adder.
If you look at the winged elementals, they have the same little horns.