Help Identifying Old (Greek?) Coin

Wildwinds is an excellent starting point for identifying ancient coins. It's database is by no means complete and comprehensive, but it's got most of the major types of ancient coins.

Unfortunately, yours isn't included on Wildwinds, because it was issued by a culture which, while influenced by Greek culture, was outside of it. It's from the Kushan Empire, which thrived in what is now northern India and southern Afghanistan in the first to third centuries AD.

The key identifier here is the little trident-like object in front of the horse, and again behing the king's neck ont he obverse. This is the king's "tamgha", a tribal badge often used in Central Asia and which functioned much as cattle brands do in the American west: everything a particular ruler owned (including the coinage) was stamped with his personal tamgha. This is the tamgha of a Kushan king known as Vima Takha (or Takto) also known on his coins as "Soter Megas" (Greek for "Great Saviour"). Click on the link to the Wikipedia page for this ruler and you'll see a couple of coins that more or less match yours.

For coins of ancient Asia, the best site I've found is grifterrec. Click on Kushan, page 2 (Soter Megas / Vima Takto), to see numerous different types of coins issued by this ruler.

The chronology of Kushan kings and coins is still poorly understood, and different sources have different dates for the various coins and rulers. Grifterrec says Soter Megas reigned 80-100 AD; Wikipedia has him 80-90 AD; my reference book has him down as somewhere between 55 and 105 AD. I'm not sure which (if any) is "more correct".

I don't like the pale green colouration that's showing on this coin; perhaps it's simply traces of the original patina the coin would've had, but if it actually is that pale green colour, then it looks like it might be bronze disease. Keep a close eye on it; if it seems to be spreading, slap it into olive oil for a month or so.