I won't deny that Heatran resembles a frog in some ways, certainly more than any other Legendary Pokémon. But that doesn't make Heatran a frog. The biggest refutation I have is his horns. Ironically, he looks more like a "horned toad" (a horned species of LIZARD) than a frog. And on that note looks similar to the Spelunky 2 Horned Lizard:
https://static.wikia.nocookie.net/spelunky/images/0/07/Horned_Lizard_Link_S2.png/revision/latest?cb=20220114054533
I am also inclined to dispute the tail claim. How is that not a tail? Some frogs have tails, FYI, though none look like that. It may not be, which is why I'd say Heatran's body more closely resembles a slug in that pointy manner. Which I know may be redundant because we have Slugma.
...
Heatran's preferred mode of movement is not hopping. Heatran walks in every media I've ever seen him in. Maybe this is different in Z-A (I haven't played Mega Dimension yet) but this is true in the anime and Scarlet/Violet. Almost any Pokémon is CAPABLE of hopping.
It's one of the more peculiar Pokémon as its design doesn't hint at any particular inspiration besides lava and armor. And the vague mythological idea of a volcano monster/dragon. Its species is "Lava Dome" and its name is "Heatran" in all languages. It conceptually resembles Divine Beast Vah Rudania from Breath of the Wild, ALSO a lizard!