Found and tamed several llamas. All can be equipped with carpet, but only a couple of them responded when clicking with a regular chest. Others just show the ride prompt. I thought it was a one-off bug, but I bred a couple of them and tamed them, but they still did not show the equip chest prompt as expected when using a chest on them. I also checked the health level of the llamas just in case that was a factor. It did not matter whether they were at full health or not.