Again, did you read what was sad? After all you did quote it.

So unless you can provide some evidence in one of the works of Robert E. Howard in which he describes wolves being of a size that could be saddled and mounted then there is no evidence that the dire wolf (as you have provided as an example) was not already extinct.

From the story Beyond the Black River:

A shuddering gasp swept over the village and men crowded hastily back, jamming one another between the huts. Balthus felt the short hair stir on his scalp. The creature that stood in the gate was like the embodiment of nightmare legend. Its color was of a curious pale quality which made it seem ghostly and unreal in the dim light. But there was nothing unreal about the low-hung savage head, and the great curved fangs that glistened in the firelight. On noiseless padded feet it approached like a phantom out of the past. It was a survival of an older, grimmer age, the ogre of many an ancient legend—a saber-tooth tiger. No Hyborian hunter had looked upon one of those primordial brutes for centuries. Immemorial myths lent the creatures a supernatural quality, induced by their ghostly color and their fiendish ferocity.
The beast that glided toward the men on the stakes was longer and heavier than a common, striped tiger, almost as bulky as a bear. Its shoulders and forelegs were so massive and mightily muscled as to give it a curiously top-heavy look, though its hind-quarters were more powerful than that of a lion. Its jaws were massive, but its head was brutishly shaped. Its brain capacity was small. It had room for no instincts except those of destruction. It was a freak of carnivorous development, evolution run amuck in a horror of fangs and talons.
This was the monstrosity Zogar Sag had summoned out of the forest. Balthus no longer doubted the actuality of the shaman’s magic. Only the black arts could establish a domination over that tiny-brained, mighty-thewed monster. Like a whisper at the back of his consciousness rose the vague memory of the name of an ancient god of darkness and primordial fear, to whom once both men and beasts bowed and whose children—men whispered—still lurked in dark corners of the world. New horror tinged the glare he fixed on Zogar Sag.

So, technically speaking at least a couple sabre-toothed cats still exist but in any “civilized” lands they had not been seen in centuries and only the power of a Shaman of Jhebal Sag was able to summon one out of the depths of Pictland. I find it unlikely that this would be an unlikely candidate for a mount.