If a lens has a collar, then it's a sign you should support the weight of the lens & body combination by the lens.

• When handholding the lens & camera, always keep your left hand holding the lens (i.e., holding the zoom or focus ring, or holding the barrel if you are autofocusing).

• When using a tripod or monopod, use the collar's tripod mount, not the camera body's tripod mount.

Some of the smallest collar-equipped lenses are possibly OK to carry and hold just by supporting the body, but realize that lenses with larger glass further from the body put lots of strain on the camera's lens mount.

For that matter, is it even a good idea to fit an 8 Kg lens on a bottom-of-the-range DSLR body in the first place?

In that case, you're really mounting the camera on the lens, rather than the lens on the camera. Plenty of wildlife and bird photographers use lower-end DSLRs with crop sensors on large telephoto lenses, to give them extra "reach".

But does it mean the lens can only be used on a tripod? As in, if you use it without a tripod, the weight will snap the camera in half or something?

You probably won't snap the lens mount in half, but you could bend it if you mounted a heavy lens to the camera and didn't support the lens.

Common sense applies here. If the camera and lens are awkward to hold by the camera body alone because the length & weight of the lens strains your wrists, then that strain is also felt by the camera's lens mount. If the system feels more balanced when holding it by the lens, then it will also be more balanced when mounting the lens to a tripod rather mounting the camera body to the tripod. If it feels wrong, then it almost certainly is wrong.