I don't personally know the answer, but exploring my way through EDICT:
a) In the definition, sense 3 - counter for horses - is listed only as " only" (so not ); it is also marked as an archaic term.
b) Skimming the examples page for , I could only find one example using a counter:
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> Only four horses competed in the race.
**Edit:**
Actually, I found another one as well:
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> He exchanged his cow for two horses.
So both of the examples use for the counter.