[skosch]

Those are cancers caused by viruses, and it's the viruses getting transmitted.

On the other hand, clonal transmission refers to the cancer cells themselves leaving sick individual A, entering healthy individual B, and continuing to reproduce there.

[apathy]

You could use transmissible preleukemia (eg CHIP in allo transplants) as an example if you wished.

Direct unassisted clonal transmission in humans seems likely but, as you noted, it hasn’t been documented to the extent that Tasmanian Devil facial tumors have.

Warts are a corner case. I’m not sure whether it’s been determined if some hosts end up increasing the fitness of the shed cells. If so, that’s quickly heading towards a globally transmitted precursor lesion.