Eusebius I Pont(ifex) Max(imus) = Eusebius I Pope
"Pontifex Maximus" is the epithet of the Pope. There was also one pope Eusebius (shortly occupied the seat in 310 AD).
https://en.wikipedia.org/wiki/Pope_Eusebius
He's not the same person as the saint that ZacUK found.
The reverse of the medal makes me think that is was a decoration which was mounted in a cavity in wooden furniture, perhaps adorned with more, similar medals of other early popes.