(this is in pokemon brilliant diamond so getting gastly is very difficult and misdreavus is shining pearl exclusive)
i tried breeding a misdreavus and gastly because they’re the same egg group, but it didn’t work and i had no idea why

turns out: they were both female o they couldn’t breed anyway

the gastly was also three times higher level than the misdreavus, maybe that had something to do with it

oh wait, i have an idea! why don’t they just make a bone marrow baby? (they don’t have bones)