Dogs, for instance, do they have a known ancestor who was not a meat-eater?
All animals evolved from the simplest single-cell ancestors. That's just as true of the carnivores as any other clade.
Dogs are actually considerably more omnivorous than wolves, even though they are subspecies of the same species:
Canis lupus lupus and
Canis lupus familiaris. When a few adventurous (or simply lazy!) wolves decided to try joining human communities, it's likely (although not universally accepted by either biologists or anthropologists) that they were attracted by the middens (a fancy word for garbage dumps) near our camps. Our garbage contained plenty of bones and offal to satisfy the appetite of a carnivore, but it was also full of vegetable matter. Eventually their physiology adapted slightly to make it easier to extract the calories from plant tissue. But more importantly, the protein content of plant tissue is significantly less than in animal tissue, so these proto-dogs had to adapt to a low-protein diet.
In warm-blooded vertebrates, the brain is the organ that uses the most protein. So dogs slowly evolved smaller brains to subsist on their new diet. That's the main difference between wolves and dogs. Other modifications are primarily examples of
neoteny, the retention of juvenile traits. Baby wolves bark, wag their tails, chase sticks and play roughhouse, but they abandon these behaviors in adulthood. Dogs do these things until the day they die, and these are behaviors that endear them to us and cement the partnership between our two species.
As you noted, other carnivores have adapted to an omnivorous diet, notably bears. The panda (a species of bear) has actually evolved into a complete herbivore. Raccoons are also not fussy eaters and enjoy eating our garbage.
Regarding hyenas, yes, they are a clade of
feliforms. The only other feliform clade includes the various species of cats. The other carnivorans--bears, raccoons, dogs, weasels, pinnipeds, and a couple of other families--are all caniforms.
My guess, and I admit that it is only a guess, is that most carnivores developed from omnivores. It’s hard to see an herbivore, say a sheep, turning into a carnivore, whereas there are instances of omnivore’s being superb predators (bears!).
Then take a second look at the cetaceans, every one of which is a carnivore--assuming you accept the mysticeti (baleen whales) grazing on krill (which are animals) as a carnivorous diet.
The entire clade of cetaceans is descended from a primitive hippopotamus, which is undeniably a herbivore--an
artiodactyl or even-toed ungulate like goats, cattle, deer, camels, giraffes and many other well-known grazers. The cetaceans and artiodactyls have recently been combined into a single clade:
Cetartiodactyla.