@Zennistrad  
Abuse is abuse. If I kick a innocent boy or an innocent dog, I’m still an ass.  
Spike is not treated like a dog in show, so why treat him like a dog in the movie. And the excuse that it makes the abuse look less harsh is bull, because if the writers cared about that, they would have stopped putting him in slapstick situations by now. The proof of magic could have been substituted by an object. Different species thing doesn’t make much sense because Equestria has dogs in their universe. What do they turn into? Dragons? And toy marketing doesn’t even have that excuse, as there is no dog Spike toy (Same goes for Flash’s character). All in all, there was no legitimate reason to make Spike a dog, and it has been the stupidest decision the show has made yet.

@Zennistrad  
Nah, she could’ve just said he’s her little brother, and that since they’re both new, they’re looking around the place together. Not to mention that Spike being able to go on about by himself could’ve probably saved Twilight some trouble with Snips and Snails.

@Background Pony #AACA
 
It doesn’t look nearly as harsh in hindsight, is what I’m saying.
 
Plus human!Twilight spend all her around with a little boy half her age who isn’t related to her would look pretty weird.

@Zennistrad  
And not a dog?  
So either you’re watching a TV show about grown women subjugating a young boy to all sorts of violent slapstick, or you get the ASPCA on your ass.

@Zennistrad  
Ironically, he didn’t get much of that in the movie…so it could’ve been fine.

@Background Pony #1133
 
Plus all the slapstick he gets subjected to would look really horrible if he were a little boy.

@Background Pony Number 17  
Because they needed a way for everyone to believe in Twilight’s story.  
No matter how ridiculous your tale sounds, a talking dog gives you credibility.

And people still wonder why they turned Spike into a dog in the EQG movie.

MENTLEGEN.