If you want to evolve a Stantler into a Wyrdeer in Pokemon Legends: Arceus, then you'll need to know exactly what to do in order to trigger the evolution criteria. It's one of the more complicated processes, which involves using a certain move a specific number of times. But don't fret, we're here to help.
While you're playing through the story you'll get to know Wyrdeer pretty well regardless of whether you evolve one too, as it's one of the special Pokemon you'll unlock that can be used as a mount.
But, this Normal / Psychic Pokemon is also a boon to have in your party, so here's how to evolve a Stantler into Wyrdeer so you can use it in battle.
1. Find a Stantler
The first hurdle with getting any evolution is to find the base Pokemon - in this case Stantler. Handily, Stantler is available in several Hisuian terrorities. You'll find it in the appropriately named Deertrack Heights in the Obsidian Fieldlands, Glacier Terrace in Alabaster Icelands, and Wayward Wood in the Coronet Highlands.
2. Make sure it has the move "Psyshield Bash"
Once you've got your chosen Stantler, you'll need to make sure it has the move Psyshield Bash. It's this move that is intrinsically linked to evolving Stantler into Wyrdeer. The description says it is used for "cloaking itself in psychic energy" so maybe that's the secret sauce.
But, you'll need to make your Stantler use Psyshield Bash in Agile Style 20 times in order to make it capable of evolving. Your Stantler also needs to be at least level 31 to evolve.
If your Stantler doesn't have Psyshield Bash, it might be that it has already learned it but not added it to its rotation in the wild. To check, go to your Stantler in your party, press A, and then go to Change Moves. You may well find Psyshield Bash in here and be able to switch it in.
3. Evolve your Stantler into Wyrdeer
Once it's done the required about of Psyshield Bashes, it's then just a case of hitting X on your Stantler to evolve it into Wyrdeer.