As of Generation IX:
Pichu Kyogre Groudon Cherrim Shellos Gastrodon Dialga Palkia Giratina Shaymin Darmanitan Darmanitan-Galar Tornadus Thundurus Landorus Keldeo Meloetta Greninja Meowstic Aegislash Xerneas Hoopa Wishiwashi Mimikyu Magearna Toxtricity Sinistea Polteageist Eiscue Indeedee Morpeko Zacian Zamazenta Eternatus Urshifu Zarude Ursaluna Basculegion Enamorus Oinkologne Maushold Palafin Dudunsparce Gimmighoul Poltchageist Sinistcha
This list excludes:
To see these other categories, as well as all Pokemon with more than two forms, please see Bulbapedia's list of Pokemon with form differences.
Just two forms? Well that would be:
Giratina Shaymin Cherrim Basculin Shellos Gastrodon Darmanitan Meloetta
However, there are Pokemon with more than two forms. They are:
Rotom Deoxys Unown Deerling Sawsbuck Burmy Wormadam Castform Arceus
