Definitely a trio imitation. As for why the Pokédex says nothing, well, that's preprogrammed, altering the wild Pokémon data never changes anything. Say you make Cyndaquil wild on Route 1, by hacking the game's ROM on a computer or something. I've done it, then looked at the entry. Nothing indicates its availability in an odd area unless the Pokédex itself is reprogrammed.
Don't ask me how they programmed Latios/Latias and the Beasts, though.