What is the name of the equipment which attaches a lens to tripod?

The far end of my 70-300 mm lens moves down a bit on the tripod after composing my shot, I think, because it is a long lens, the weight of the lens makes it move down. So, I have to consider in advance that it will move down a bit and compose accordingly. I have seen an equipment in a video which attaches the lens instead of the camera to the tripod and because it is a more balanced situation the far end of the lens stays stable and doesn't move down. But I don't know what it is called and that's why I can't find and order it through internet So, I would be happy ,if anyone knows this device and share its name.

Or,if there are other ways to keep a long lens stable on a tripod , I would be happy to hear.

Thanks for your help in advance