What could allow dogs of this fictional breed to not have mobility issues (given they weigh more than any rl dog breed)?

[GodzillaLouise]

Let’s cut to the chase. In my Godzilla/Ultraman fanfic world, male Voswuinian Mastiffs are a dog breed, bred originally by Voswuinians from the planet Lyrial (long story, but just know that Voswuinian Mastiffs are, in fact, actual dogs), which can weigh anywhere from 400 to 520 pounds and stand at up to 5 feet 5 inches at the shoulders as adults and have a physique visually and muscularly somewhere between that of a Great Pyrenees and an English Mastiff. Female Voswuinian Mastiffs are slightly smaller, but are still of considerably giant size for any dog breed.

However, despite their immense weight and build, Voswuinian Mastiffs exhibit considerably less mobility issues than most dogs of this size. Even the most outright elderly/geriatric Voswuinian Mastiffs are just as mobile as when they’re adolescents.

For example, Zageerla, a Voswuinian Mastiff in one of my stories, is planned to be anywhere from 9 to 12 years old, though I haven’t decided where his age lies in that range yet. Either way, that would make him an older dog, yet the plans are for Zageerla to show the audience multiple times that, despite his age and large size, he’s quite an agile and mobile dog. I’d be surprised if there’s not at least an episode of my series where the climax involves Zageerla doing something drastic in an effort to protect Veruze (his owner/caretaker).

Now, I’m mainly focusing on the physical side of things: what about Voswuinian Mastiffs is it that allows them to not have the problems that other big dogs have with mobility when getting old?

[Flavia]

In a world where Godzilla exists, I don't think you have to worry about that. Just add a pinch of handwavium.

[Visions1]

Maybe they don't age, but still die. Not sure what that would be. Probably rely on Crispr or some other far out gene business.
Or, maybe they normally do pretty well until they turn around 20 - and then, it's just a total systems collapse over a month or so.

[GodzillaLouise]

Maybe they don't age, but still die. Not sure what that would be. Probably rely on Crispr or some other far out gene business.
Or, maybe they normally do pretty well until they turn around 20 - and then, it's just a total systems collapse over a month or so.

What would the “they don’t age but they still die” route entail, in terms of their growth in size? Would it mean they don’t start off smaller when they’re ‘younger’ or would they still grow overtime?

[Visions1]

Well of course they'd start of smaller. Think of it like with lobsters - the animal grows into an adult, but never becomes elderly. It stays young.

[elemtilas]

Two things come to mind!

First, we know that modern dogs and their various breeds all eventually derive from an ancestral wolf-dog. Think of the descent lines like one or more cone shaped cornucopias of many wires: sometimes the wires split off, sometimes the wires twist, sometimes they connect, sometimes they're cut short. But as we move backwards in time, there are fewer and fewer wires, until there is eventually one. These wires represent the bloodlines and their crossing nodes. Each of these nodes also represents the various good, bad, and indifferent traits that modern dogs have.

The first thing I would consider is creating a new lineage or family of lineages that specifically lack the bad traits you're concerned about and favour the good traits you want. As a matter of worldbuilding, this could be expressed in terms of myths or legends regarding semi-divine, wolf-running, dog-breeding demigods; it could be a matter of the ancient Lore of a guild; it could also be a matter of technological discovery that reveals something special about these mastiffs that don't suffer what other breeds do.

Second, we know that canids are related, though not closely, to ursids. In our world, think of ursidae and canidae as two long separated descent lines, each with their own wire baskets of descent.

The second thing I would consider then is that your focus dogs are actually bears. Perhaps you could express this mythologically through two brothers, one of them a wolf-running dog-breeding demigod and the other a bear-running dog-breeding demigod. If interbreeding is allowed, perhaps you could establish that modern dogs are mostly descended from wolves, but that some breeds are descended from monstrous bearwolves that, thankfully, no longer roam the Earth.

Bears can be huge and seem to be pretty mobile and agile even up to three quarters of a ton.