Throughout the Horus Heresy novels "Prospero Burns" by Dan Abnett and "A Thousand Sons" by Graham McNeill, the phrase, "There are no wolves on Fenris," is mentioned throughout these novels. There are cryptic references by none other than the Thousand Sons Primarch Magnus the Red as to the unique nature of the genetics on the world of Fenris and the Canis Helix sequence of genes within the Space Wolves' gene-seed. It has already been mentioned in various sources of official canon that the Canis Helix is responsible for the peculiar lupine traits possessed by the Astartes of the Space Wolves Chapter. It is also hinted that perhaps the Fenrisian Wolves are actually Astartes whose genetic alteration has spiraled out of control, reshaping them into bestial creatures. This would explain how and why these supposed "wolves" fight alongside the Space Wolves in battle and readily obey their Astartes masters in the heat of battle, perhaps retaining some fragment of their former lives and sense of duty.
But this does not explain how "wolves" have existed on Fenris since before the first Space Marines of the VI Legion came to Fenris. It is generally known that during this earlier age when Mankind colonised the galaxy that humanity possessed advanced genetic engineering technology. For those early settlers that colonised the inhospitable, icy Death World of Fenris, they may have attempted to find a way to adapt themselves to the harsh environments of their new homeworld, splicing their DNA through gene-manipulation with that of Terran wolves that were more suited to the arctic conditions and climatic extremes. One could extrapolate that this would have provided the Fenrisian colonists with a fighting chance at surviving this hellish environment. In some cases this manipulation must have had unforeseen consequences.
Perhaps the first generation of "wolves" came from the original gene-tailored colonists that devolved over time until their genetic alterations stabilised, creating the first Fenrisian Wolves. When the VI Legion arrived much later during the Great Crusade in the late 30th Millennium and began recruiting Astartes from the Fenrisian population, there was the occasional destabilisation of the delicate balance which resulted in the rampant genetic mutations which resulted in a new, more potent genetic line of Fenrisian Wolves. The larger and hardier stock of Thunderwolves may be the possible genetic offshoot of these early Astartes, while the more common Fenrisian Wolf would be a result of the original colonists' gene-enhancements. The interbreeding between the two species would have resulted in the development of the lesser Fenrisian wolfkin, producing a viable breeding population. It has also been hinted throughout the novels that the Wolf Pelts and fetishes worn by the Space Wolves were actually the hides of the descendants of these mutated humans.
If all modern Fenrisians are descended from the original colonists who attempted gene-splicing with that of lupines and canines, that would mean that all Fenrisians are in some way part wolf. The process by which a new Space Wolf Astartes is created would awaken these genetic traits that already lie dormant within their bodies, resulting in amber-coloured wolf-like eyes, long canines and an enhanced sense of smell. But when these genetic traits run rampant, the result is accelerated and uncontrollable growth, giving rise to the Curse of the Wulfen -- mirroring what had occurred to the original Fenrisian colonists thousands of years earlier. In the novel "A Thousand Sons" it is mentioned multiple times that when someone fighting a Fenrisian Wolf looks deep into the creature's eyes, in that moment the horrible realisation sinks in that they are facing something almost...human.