This little guys hide in the sand, you'll need a shovel to find them.

Shovels cost 100-400 Credits depending on Tier, I'd suggest lowering to Tier-1 if you'd like to get this Plushie, it can be a pain in the butt due to being RNG.

You'll find it the Crab plush by digging up Buried loot, it's a 5% Chance of finding the Plush. Be Patient and it will eventually come to you.