Largest living species. Which is one reason why I mentioned dire wolves as an example, which may have been a tad larger and would have existed in the Hyborian Age time period.

And as mentioned earlier, although this is a low fantasy setting, it’s still fantasy. Which means that while it’s still guided by reality and historical record, it’s not strictly bound to it as demonstrated by the presence of various demonic and mythical creatures, among other things. It would be entirely consistent within this setting for there to be a species of wolf large enough and strong enough that one could potentially ride it as a mount.