Aside from Dragonite, Reshiram does not possess other forms. So, take a look at its fanmade Mega form.

The elegant yet powerful Reshiram became even more dauntless with its Mega form. Artist villi-c played with a bunch of colors to give the Pokémon an intimidating yet graceful appearance. We commend how unique and creative the artist is in creating this design. Now, Reshiram got four wings, pink highlights, and a massive jet-engine tail. Can you imagine how powerful this Pokémon will be in its Overdrive mode?
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References:
https://www.deviantart.com/villi-c https://www.deviantart.com/villi-c/art/MEGA-RESHIRAM-746749598