Here is the full list of all 53 Hidden Cats in Santa's Realm achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Snowball Fight Completed
You Found 800 Cats
Santa's Castle Completed
You Found 700 Cats
Reindeer Stable Completed
Magic Forest Completed
You Found 600 Cats
Frozen Lake Completed
You Found 500 Cats
Elven Pub Completed
Candy Factory Completed
You Found 400 Cats
Found All Cats Advanced
Finish with no Hints Advanced
You Found The Gift Cat
You Found Hadouken
You Found 300 Cats
You Found The Santa Claus Cat
You Found the Christmas Tree Cat
You Found 250 Cats
You Found Aurora
You Found Ranira
You Found 200 Cats
You Found Flocus
You Found Celeste
You Found The Elf Cat
You Found The Krampus Cat
You Found 150 Cats
You Found Zelda
You Found The Nut Cracker Cat
Found All Cats Normal
Finish with no Hints Normal
You Found 110 Cats
You Found 100 Cats
You Found 95 Cats
You Found 90 Cats
You Found 85 Cats
You Found 80 Cats
You Found 75 Cats
You Found 70 Cats
You Found 65 Cats
You Found 60 Cats
You Found 55 Cats
You Found 50 Cats
You Found 45 Cats
You Found 40 Cats
You Found 35 Cats
You Found 30 Cats
You Found 25 Cats
You Found 20 Cats
You Found 15 Cats
You Found 10 Cats
You Found 5 cats