This came in the CNG lot with the Hirkod. Only the reverse was visible, and I thought it was a much more common Iceni coin. I was mistaken!
Celtic Gaul, Aulerci Cenomani Tribe
Area of Le Mans, France
AR 1/2 unit (11mm, 0.62g)
80-50 BC
Obv: Head of Athena right
Rev: Stylized horse right, schematized Nike holding wreath above; solar pattern below (some argue a bust of Helios?)

I just love the organic, flowing artistry of the La Tene Celtic style! I hope someday to add a few more (hopefully larger) examples, but these are quite popular coins, and thus quite expensive!
The only extant history of this tribe is from the Roman perspective. Julius Caesar notes that this tribe contributed a force of 5,000 soldiers to aid the chieftan Vercingetorix in his last-ditch effort to unite Gaul against the Romans. They were all subjugated in 52 BC, and the Cenomani were subsequently Romanized.
They are not to be confused with the Cenomani in Cisalpine Gaul; another coin-issuing tribe which may or may not have been ethnically/culturally related.