They get us good sometimes. The actual largest canid was a species called epicyon which averaged ~250 lbs, but the biggest and baddest may have gotten to nearly 400 lbs. It lived and died out long before the genus homo existed, much less homo sapiens. Still, even the biggest and baddest epicyon at 400 lbs is not big enough to make a good mount. That's a very little pony. Good enough for Joe but not us.
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We have to look outside Canidae to find our dog mount. Canidae is part of Caniformia (dog-like carnivorans which includes bears, mustelids and seals) as their next closest relatives. The elephant in the room: bear cavalry. The closest thing we'll get to wolf mounts and pretty metal on its own. Pretty sure the Russians have tried it. Don't think it worked too well for them but I can understand the dream.
Bears are kind if their own thing though, right? Riding an agile, loyal giant wolf into battle just isn't the same vibe as riding a bear. Both bad ass, but different. Let me introduce the family amphicyonidae, the "bear dogs", an extinct family in caniformia which evolved 40+ million years ago but managed to survive to around 5 million years ago. As their name implies they look like a mix of a bear and a dog, with some looking more like one than the other, though generally trending to more bear-like in the youngest members of the family (with one big difference, they were all obligate carnivores unlike most bears). They were the closest relatives to canidae (dogs, wolves, foxes etc). Some of the species weighed up to over 1,700 lbs. That's riding size. So if you really wanted to ride a giant wolf like creature that actually existed and you had a time machine, I would say Amphicyon or Pseudocyon would do the trick. It's not perfect but it's closer than just riding a boring regular bear, imo.
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Of course if you had a time machine you'd probably already have the technology to just create some sort of science fiction horror Clifford the big red dog, but that's a different conversation.