I recently bought a used LeafGreen game, and it came with a save file that had a lot of Pokémon, obviously sharked, but without going into detail, one caught my attention.
There's a japanese Celebi. Name セレビィ, OT アゲト and ID No. 31121, which coincides with the Japanese Colosseum Bonus Disc Celebi. It's odd for someone to hack Japanese names (plus the game had another Celebi, with an american name/OT), let alone japanese OTs around these parts, but the Celebi having a Modest nature makes me doubt if it's legit.
Is there another way to check if this Pokemon is legit in D/P? Under which conditions Pokemon Battle Revolution displays Pokemon as bad EGGs? This LeafGreen game seems to have been used to dump clones of Pokemon in it too, as it has multiples copies of the same Zigzagoon, with same nature, gender, stats and nickname.
Give it to a hack checker. Dawnbringer/goodbar in the wifi forum.