Not Celtic, I don't think. It's actually from Kings of Thrace, Adaois. Mid 3rd century BCE. (You can barely make out ΑΔΑΙ on the reverse above the spearhead.) Notice that your reverse has the same monogram below the spearhead as on the examples linked below: HP (ligate) - Σ. Your obverse is hard to make out from the photos, and possibly too worn, but I would imagine it's the same boar head facing right that appears on all of these (in any case, the reverse is the same): https://www.acsearch.info/search.html?id=4800194 https://www.acsearch.info/search.html?id=3665643 https://www.acsearch.info/search.html?id=5064230 https://www.acsearch.info/search.html?id=5062258 https://www.acsearch.info/search.html?id=5493187 https://www.acsearch.info/search.html?id=7882151 On ACSearch you will find additional examples (maybe 40, including legend variants) by searching for combinations of relevant terms, boar, spearhead, Adaios. There are other reverse monograms possible (probably from a mint official/"magistrate"?), but yours (HP S) seems most frequent: https://www.acsearch.info/search.html?term=spearhead+Adaios https://www.acsearch.info/search.html?term=adaios+boar&thesaurus=1
