Re: Need help identifying this plant I think it is an Austrocactus coxii http://cactiguide.com/cactus/?genus=Austrocactus&species=coxii
I wonder if it could be a Mammillaria rhodantha ssp. pringlei? If so it is more cylindrical than it should be, but it shows clear signs of etioliation at the top, so maybe it has not ever been grown in strong light? Has it flowered?