Let's just keep it "sticky", rather than make it mimic-like.
CR 8? Downsizing a purple worm 6 HD and one size category yields CR 9, and I'd knock off another CR for the lack of deadly poison.
Even CR8 seems a bit high, I'd say 7.
A hill giant is CR7 and looks like it's roughly the same level of nastiness.
I'm thinking a Purple Worm could be a bit over-challenged.
As for the hiding under the treasure, at first I thought it was fine but then I roughed out an idea of how much its horde actually covers.
Let's assume CR8 with triple coins, the standard treasure table says that's a maximum of 36,000 coins (assuming three 12's on 1d12 × 10,000 copper pieces), but its more likely to be in the thousands (2d6 × 1,000 sp). Call it 10,000 coins. Each coin only covers about a third of a square inch (if not less - precious metal is dense and they only weigh 1/50th of a pound). That's 23 square feet, which isn't enough to cover a 5' square, let alone its entire body.
Some kind of treasure-accented mimicry makes more sense to me.
15,000 lb based on the purple worm.
I made it 15,000 pounds too.