I'm sure that this question has been around for a while now but whatever. How come Spike the dragon becomes a dog but the Diamond Dogs become humans? Well the first thing that comes to my mind was that Hasbro was just truing to be realistic. Yeah talking dogs, rainbow colored people but no the Diamond Dogs have to become humans not draogns. Oh well. Anyway, my theory is that they are equivalent to ponies so they become humans. Yeah that's all I got. What about you guys?

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The thing to accept is that you don't change species because of some invisible "rules of magic" when you cross a portal between universes. Proof is when CH and CP students crossed into Equestrian airspace but didn't immediately change into ponies or whatever.

The real reason you change species is because of the mirror portal itself. It's not just a raw wormhole, but more akin to the Star Trek computer. It's a processor and enabler. It makes sure when you cross worlds, you don't exit over a cliff, you don't exit underwater, you don't appear alien to the natives, you're outfitted with the right clothing and gear for survival and infiltration, etc. It's an extremely complex piece of work aside from generating a wormhole.

So Spike becoming a dog is not because he's inferior to ponies or whatever, but because the computer made an arbitrary decision that making him a dog will help the pony infiltrate better. The computer also made the arbitrary decision to make Twilight the same age as Sci Twi, and to give her student clothes, a bookbag, etc.