Mewtwo really appreciates entry hazard support from Pokemon like Primal Groudon, Necrozma-DM, Deoxys-S, and Dialga.
Mewtwo values Primal Groudon, Zygarde-C, Marshadow, and Ho-Oh for checking Steel-types like Necrozma-DM, Magearna, and Aegislash, especially if Mewtwo lacks Fire Blast.
Tapu Lele provides Mewtwo with Psychic Terrain support, increasing the power of Psystrike and negating any priority like Marshadow's Shadow Sneak that might stop Mewtwo.
Dark-types such as Yveltal, Arceus-Dark, and Darkrai deal with the Ghost-types that trouble Mewtwo, particularly Marshadow, Necrozma-DW, Lunala, and Aegislash.
Fairy-types like Xerneas and Arceus-Fairy check Dark-types, and Arceus-Fairy is also able to deal with Marshadow defensively. Mewtwo can check Poison- and Steel-types in return.
Setup sweepers such as Necrozma-DM, offensive Primal Groudon, Swords Dance Ultra Necrozma, and Extreme Killer Arceus appreciate Mewtwo's ability to weaken Pokemon such as Zygarde-C and Toxapex for a sweep.
Bulky walls such as Primal Groudon, Arceus-Water, Arceus-Fairy, Zygarde-C, defensive Primal Kyogre, and defensive Yveltal can sponge attacks aimed at Mewtwo.
Yveltal, Magearna, and Mega Scizor (if using regular Mewtwo) grant Mewtwo free and safe switches.
In short, mons that can deal with its checks are good support mons and that will be based on which coverage Mewtwo lacks. Its checks include Yveltal, Necrozma and Lunala among others especially if it lacks a specific coverage move. You can read more about it here
