To complete your Pokedex you'll need to evolve a Sneasel into Sneasler in Pokemon Legends: Arceus. While you'll get one as a special Pokemon that allows you to climb up cliffs and other vertical surfaces (apart from ice), if you want to complete your Pokedex you'll also need one in your party or pasture.
It's not available to catch in the wild, so here's how to find a Sneasel and evolve it into Sneasler.
1. Find a Sneasel
First, let's make sure you've got a good Sneasel to evolve into Sneasler. You'll find the Hisuian Sneasel in the Avalugg's Legacy and Glacier Terrace areas of Alabaster Icelands, and in Celestica Trail and Primeval Grotto of the Coronet Highlands region.
2. Obtain the evolution item, Razor Claw
Next you'll need the evolution item known as Razor Claw. It's been a staple in the Pokemon series for a while now, and there's a couple of ways to get it in Pokemon Legends: Arceus.
Firstly, it can drop as an item when defeating or capturing a Sneasel, which is super handy. Hopefully, it didn't fall off the Sneasel itself... Or you can head over to the Trade Stop in Jubilife Village. She'll sell you rare items, including evolution items, in exchange for Merit Points or MP. It's the currency you earn for collecting and returning the lost satchels you can find across Hisui.
3. Evolve your Sneasel into Sneasler
Once you've got your Razor Claw and your Sneasel in your party, then all you need to do is select the Razor Claw in your inventory and use it on Sneasle to evolve it into Sneasler, a very powerful Poison / Fighting Type. This is the only way to get Sneasler as a useable Pokemon, so make sure you do this if you want to complete that Pokedex.