Looks like I have to revive this thread with a cool rock story recently posted On the Troon Dungeon thread. Since it was pointed out that this thread has been shamefully neglected by @The Hoh Rainforest, we must do better!
One of the coolest things I've ever seen was when a sea otter approached a bunch of human fishermen bearing one of the shiny white tide-polished rocks they use for busting open clams on their bellies, and holding it out to them while looking at the fish they had caught.
One of the fishermen asks the otter if he wants to trade, and the otter actually puts the stone in his hand and taps it, like saying 'hey, this here's a damn good stone, man!' and so the fisherman gives the otter a fat trout in exchange and the otter stuffs it in his yap, totally happy with his deal.
Mustelids are my favourite animals and otters top that list.