I'm positive, I did not manage to obtain a ditto in my original Sapphire game. I bred a lot on that game, and I am adamant that my old Castform would breed with anything I put with it, so long as it was of another gender. This is how I managed to get the "baby" pokemon (Pichu, Cleffa etc), and I am positive I bred Absol, Zangoose, the three starters, Electrike, among many other pokemon.
I'd just leave Castform in the daycare, and just swap the other pokemon
Unless... this was a glitch, and I never realised