The Xeno Arena update has suddenly converted No Man’s Sky into a monster-collector adventure, and at the top lies the S-class pets. All pets who fight for you will have different tiers. While you can upgrade all of them, it’s always beneficial to find S-tier pets from the get-go.
How to find S-class Xeno pets in No Man’s Sky
For starters, you’ll need to clear a fair amount of in-game content before you’ll find a quest that will guide you through the basics of Xeno battles.

There are three tutorial battles to complete as part of the Xeno Arena quest. Once those are done, you’ll be able to explore and collect creatures of your own freely. Go to any planet you want, but there must be a life form on it. Wait for a couple of minutes for the animals to load on the planet, and then you’ll need to use your scanner.
Once the scan is complete, you’ll be able to identify the animal’s tier. Continue this process until you find one with S-tier class as its base. This could also be one that was discovered by another player and scanned on the planet before.

Approach the animal, and you’ll get the option to feed it. Provide food for the animal, and you’ll be able to add it as a companion. It can now be used for fights in Xeno Arenas once you’re aboard the Anomaly.
S-tier Xenos are not perfect. Some of them can be incredibly underleveled when you find them. Naturally, they’ll still be weak. However, they will grow quicker and better compared to low-tier companions. That said, you can do fine without setting up a team of only S-tier companions.