You have to choose where to play your Carrot Seeds. This guide on Choose Field Covered In Snow Or By The Graves In Pokemon Sword & Shield Crown Tundra will explain the differences between the two outcomes on whether you choose to use the snowy field or the field by the graves.
During the 1st Legendary Pokemon clue story you learn about a missing steed. During your adventure you learn that you have to find some Carrot Seeds and then find special locations to plant them, so you can attract the steed and catch it as your own Pokemon. There are two choices you can make here. You can plant either Iceroot Carrots (Ice) or Shaderoot Carrots (Ghost/Psychic).
Choose Field Covered In Snow
As you have probably guessed already, the outcome determines the type of legendary Pokemon you can catch. If you choose to plant the Carrot Seeds in the field covered in snow, you will eventually have the opportunity to catch Calyrex as a Psychic & Ice dual type Pokemon.
Choose Field By The Graves
If you choose to plant your Carrot Seeds in the fields by the graves, you will eventually be able to catch Calyrex as a Psychic & Ghost dual type Pokemon.
Ultimately, both Pokemon are very strong. It really depends on which visual you prefer and how the types will fit into your teams.