On the Origin of the Kantonian Ponyta: Actually, fossil evidence suggests that the reverse is true. Or, well, sort of. From what we can tell, ponyta and rapidash as we know them emerged on the Eurasian continent, which Kalos is part of. Eventually, those pokémon migrated eastward. Some managed to get to Kanto to develop an ancient regional variant, but this variant actually wound up going extinct, either on its own or through assimilation into the more common variant. The common or continental variant, on the other hand, arrived in Kanto and the rest of Japan from China and Korea sometime in the fourth century. Meanwhile, the Kalosean variety is descended directly from the first ponyta that appeared on the Eurasian continent, and it was domesticated by the Kalosean people long before Kanto had seen them.
On Armor: It’s possible, only in the sense that anyone would be uncomfortable if they had to dress up in full costume every single day. But joking aside, the armor you see in a renaissance faire is often crafted specifically for both historical accuracy and the comfort of the pokémon wearing it, so while it might be heavy, it’s usually not uncomfortable for a rapidash to wear it so long as it’s taken off between bouts.
Also, the armor has some level of practical use too: even though renaissance faires probably don’t use weapons that are actually designed to kill a person, getting struck with it while running at a full rapidash’s gallop will still hurt, but the armor helps take the brunt of the strike, at least.
On Stripes: Pigmentation can be a funny thing sometimes, so actually, yes! You may be looking at faint stripes. Either that, or it may be uneven brush strokes or an uneven trim, depending on where you’re noticing the stripes and how you groom your ponyta.
The Pokédex According to Bill 