There's at least a couple mechanisms. The most obvious is heredity—certain alleles of a particular gene that normally controls growth, or expresses some surface

There's at least a couple mechanisms. The most obvious is heredity—certain alleles of a particular gene that normally controls growth, or expresses some surface protein, mutate, leaving descendants with a greater risk of cancer. BRCA4, for example, has some variations that all but guarantee breast cancer. Mutation can happen in both somatic (body) cells and in germ-line cells; when it happens in the latter, the mutation can be passed on to offspring. Oncoviruses are another transmission vector. HPV, or human papillomavirus, is a virus that causes warts (of all kinds) in humans, and also causes several kinds of cancer, including cervical and esophageal cancers. A third way would be a pretty rare thing in humans, but is thought to be behind some of the canine cancers you mentioned: direct transplantation of cells. The tasmanian devil is on the edge of extinction thanks to a facial cancer they get from each other through this mechanism.