I don't know about magmar but it could be cause magmortar has a cannon which it shoots things outta so it makes more sense I guess
commented 5 hours ago by ~BulbaBruh™~
Maybe as a Magby, out of it's five egg moves, four of them are Physical moves. But because Magmortar's movepool itself has more Special than Physical moves, it switches to Special.
Also, Magmar came out in gen 1, which happens that all Fire type moves will be special.
Magby came out in gen 2, with a contrast of a physical mon(or is it just me). Magmortar came out in gen 4 with a contrast of a special mon. Also, since Magmar at first, had higher special attack, Game Freak made it that it would be easy to trade over your old Magmar from gen 1 or 2 to gen 4 and evolve it without changing it's Physical > Special or Special > Physical sets
Magby learns more Physical moves than Special moves by transfer from another generation
commented 2 hours ago by ★~ProfDelldell~★
Other than that evidence, going into Magby’s gen 4 Pokémon moveset, Magby learns more Physical moves than Special moves through TM. During gen 3, Magby could transfer some moves to gen 4, and none of those moves were Special. Magby learnt lots of Normal type attacks through breeding, TM and HM. And at that time, all these moves were considered Physical moves. Also, the item that allows Magmar to evolve when traded, the Magmarizer, has a description that also might explain how Magmortar got its cannon and high SpA.
A box packed with a tremendous amount of magma energy. It's loved by a certain Pokémon.
The box seems to have a hole on it. Like how Magmortar’s blaster is like.
Magmortar’s Pokédex entry also says:
It blasts fireballs of over 3,600 degrees Fahrenheit out of its arms. Its breath also sears and sizzles.
That means it also inherited the lava from the Magmarizer so now it is special.
Or it could just be
Game Freak logic
I’ve hoped I’ve helped!