Old Roman Medallion/Coin?

Appears to me as Saint George or a knight slaying a dragon. Couldn't find the exact one, so just a guess.

It's not a coin, and not Roman.

St George slaying the dragon is a national symbol for several countries (including Britain), but it is also a popular motif for religious medals. St George is a patron saint of soldiers, so badges and medals featuring him are often worn by Catholic or Orthodox soldiers when going into battle. They are still making similar medals today; just type "saint george medal" into Google and you'll find hundreds of people who will happily sell you one.

I assume it's quite large? If it's large, and it is uniface (no design on the other side), then it might have been designed for attaching to the saddle, bridle or other piece of kit of a war horse, rather than intended to be worn by a person.