Formians are ant-centaurs, insectoid creatures native to the plane of Arcadia. Some have moved to other planes, especially Mechanus. Arcadian formians are substantially more peaceful than their aggressive, expansionist cousins elsewhere.
Sources:
Planes of Law
Tales From the Infinite Staircase
Manual of the Planes (D&D 3.0)
Fiend Folio (D&D 3.4)
Monster Manual (D&D 3.5)
Beyond Countless Doorways (d20, Malhavoc Press)
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