So ワンコイン refers to a 500円 coin, but is there other names for the other coins?
Also in conversation does ワンコイン refer to any specific currency or only to 500 yen (as in the sense of being a default).
I'm not sure how SE works and why there are only comments with answers and no actual answers...
However, ワンコイン, as mentioned by Dave and others in the comments, is not a term for any particular coin, but just a sales practice by any shop. Whenever they sell items where you can pay using just one coin, then it's a sales point they'll put on signs.
It could apply to any of the coins. So if a shop had everything for 100 yen, or 10 yen, 5 yen, or 1 yen, then they could say ワンコイン.
I have never heard the term used in conversation, at least not in reference to currency. It would only be used when talking about a shop or sale.
I agree with Dave M G. There are only 1, 5, 10, 50, 100, and 500 yen coins, and among them, probably 100 and 500 yen are the only price that will be practical for selling thing under a unified price. So you will probably not see anything other than 100 or 500 yen being referred to as
ワンコイン, but it should mean any coin. I don't agree, but if you do feel that
ワンコイン does not refer to 100 yen, then it is probably because there is term
100円ショップ, and 'hyakuen' (4 mora, 3 syllable) is shorter and more precise than 'wankoin' (5 mora, 4 syllable).