The best we can say is that the mon is legal (has traits that are naturally obtainable). It's never possible to say with 100% certainty if it's legitimate or not unless you have proof of the Pokémon being obtained (in this case, a video/etc. of the mon being downloaded). And many communities have different definitions of what legitimate means anyways.
All that being said, error code 10015 displays when you're trying to deposit a Pokémon that should have a HOME tracker but doesn't, or had some of its values edited (hacked) after a HOME tracker was assigned. If this is the only Pokémon you were trying to deposit into HOME when you got that error code, it sounds like whoever traded this to you hacked it into SV directly rather than transferred it up from Gen 7 through official means, so it's likely a hack.