Thrace was one of Macedonia's traditional enemies, sitting as it did to the north of Macedon. As with many of Macedonia's enemies, they were conquered and their best troops duly pressed into Macedonian service. Peltasts owe much of their fighting style to that of the Thracian light infantry before them.
Thracians were justly famous for their light troops, and whilst regularly fielded as peltasts, they were more than capable of joining the thick of the action with their wicked curved-bladed rhomphaia swords.
Other trademarks of Thracian peltasts are the fox-skin Phrygian cap and long, geometrically-patterned cloak, known as a zeira. They will certainly make for a distinctive addition tom your armies.