I have one ditto in the day care and I thought it would be nice to know what Pokemon it cant breed with. Otherwise I'm just gonna be going around while its in the day care with something and I'm gonna wonder why I havnt got an egg yet XD
Ditto can not breed with
- Legendaries (breeding manaphy gives a phione egg)
- nidorina and nidoqueen
- baby Pokemon (ex. pichu, cleffa, igglybuff, etc.)
Source: PokemonDB breeding and egg group guide
Ditto cannot breed with legendary Pokemon since they are in the undiscovered egg group.