Because I remember someone telling me that T-tar is the only fully evolved Rock/Dark type even after two decades I went to see when the last time a fully evolved pokemon had every type. I counted megas which changed types as well as regionals.
Not counting megas/primals
Grass/Fairy->Gen 7
Rock/Fighting->Gen 6
Bug/Steel->Gen 5
Ghost/Dark->Gen 4
Fire/Ground->Gen 3
Fighting/Psychic->Gen 4
Dragon/Fairy->Unused
Not counting regionals
Normal/Ghost->Unused
Water/Dark->Gen 6
Grass/Electric->Gen 4
Grass/Fighting->Gen 6
Ice/Rock->Gen 6
Poison/Dark->Gen 4
Psychic/Flying->Gen 5
Electric/Psychic->Unused
Ice/Steel->Unused
Ice/Fairy->Unused
Not counting misc. alt forms
Water/Grass->Gen 3
Grass/Rock->Gen 3
Fire/Ice->Unused
Fighting/Steel->Gen 5
Fire/Flying-Gen 6
Ghost/Flying->Gen 4
Psychic/Dragon->Gen 3
Fire/Electric->Unused
Water/Electric->Gen 2
Grass/Flying-Gen 3
by theguyinyourwall