From TR’s Through the Brazilian Wilderness (1914):
The dogs were a wild-looking set. Some were of distinctly wolfish appearance. These, we were assured, were descended in part from the big red wolf [maned wolf] of the neighborhood, a tall, lank animal, with much smaller teeth than a big northern wolf. The domestic dog is undoubtedly descended from at least a dozen different species of wild dogs, wolves, and jackals, some of them probably belonging to what we style different genera. The degree of fecundity or lack of fecundity between different species varies in extraordinary and inexplicable fashion in different families of mammals. In the horse family, for instance, the species are not fertile inter se; whereas among the oxen, species seemingly at least as widely separated as the horse, ass, and zebra—species such as the domestic ox, bison, yak, and gaur—breed freely together and their offspring are fertile; the lion and tiger also breed together, and produce offspring which will breed with either parent stock; and tame dogs in different quarters of the world, although all of them fertile inter se, are in many cases obviously blood kin to the neighboring wild, wolf-like or jackal-like creatures which are specifically, and possibly even generically, distinct from one another. The big red wolf of the South American plains is not closely related to the northern wolves; and it was to me unexpected to find it interbreeding with ordinary domestic dogs (pg. 74).
Roosevelt was wrong about the origins of the domestic dog. We know that domestic dogs are just a form of wolf (Canis lupus). However, at the time, virtually everyone believed that various types of jackal, even the ones that have never been known to inbreed with dogs, were in the mix. African wild dog (Lycaon pictus) and dholes (Cuon alpinus) have also been claimed as possible ancestors of the domestic dog, but no one has produced a hybrid from a domestic dog and these animals. We now know that dogs, including New Guinea singing dogs and dingoes, fit within the wolf species and can interbreed with coyotes, golden jackals, and Ethiopian wolves.
There have always been persistent rumors of other wild dogs interbreeding with dogs. The most common unsubstantiated dog/wild dog hybrid is a hybrid between a black-backed jackal or a side-striped jackal, which both belong to the genus Canis, but no confirmed hybrids between these species and domestic dogs have ever been documented. However, in the nineteenth century, there were many claims that red foxes had crossed with dogs. Such crosses, if they ever existed, would have likely been sterile, because foxes and dogs have vastly different chromosome numbers.
Both of hybrids between dogs and the endemic African jackals of foxes are probably urban legends.
However, I have also come across supposed crosses between domestic dogs crab-eating foxes, which are a South American wild dog species. South American wild dogs, some of which are called foxes, are actually much more closely related to the true dogs in the genus Canis than they are to the red fox and its closest relatives.
I don’t know if the existence of these animals has ever been verified, so I am very skeptical.
But there is another possibility: the Brazilians could have had a domesticated maned wolf that could be used as a hunting dog.
The natives of Tierra del Fuego had domesticated the culpeo, another South American wild dog that is sometimes called a fox or zorro, and may have used them to hunt otters.
However, if look at the context of Roosevelt’s description of the dogs, they were being used to hunt jaguars.
I know of no single account of a maned wolf approaching a jaguar for any reason. Maned wolves are not really equipped to hunt large game and are not competitors with the jaguar in any way. Further, they don’t hunt in packs, which they would have had to do if they were going to cause a jaguar any trouble. Domestic dogs are better equipped to chase jaguars because they do have a pack hunting heritage that they receive from the wolf, but it is unlikely that any supposed domesticated maned wolf would be a pack hunter that would readily pursue a jaguar.
My guess is that Roosevelt saw some particularly rangy domestic dogs with reddish-colored fur that the Brazilian claimed came from the maned wolf. They likely never saw the dog mate with the maned wolf. It may have been nothing more than a claim that was used to sell the puppies.
I would love for this story to be true, but in light of what is already known about hybridization within the dog family, I am very skeptical.