They could have had Spike be Human, claim that he's Twilight's "Little Brother", claim that he's a smart/gifted kid who skipped a few grades(and he's bound to have learned a lot being Twilights assistant and all), and have Spike teach Twilight to use her fingers/hands and walk like a biped since she'd be too weak to Horsey-back-ride him on her back despite the fact that he'd be at least half her height, and have him be exposed to magic and regain his dragon claws, fire-breath & teeth that can chew gems, but instead they wanted Spike to become something small enough to fit in Twilight's backpack, so they went with a dog, it just feels like they weren't even trying to get Spike to reach his higher potential.