You have Sonic and you have Pokemon, A WINNING COMBINATION! Let me sho ya somethin'!
Sonic has a Shaymin (in sky form). The reason: They're both hedgehogs (well, one of them, anyway). They're both into speed and they seem to both share the same personality as they fight for their friends, because of their loyalty, and laughs at the face of danger.
Tails has Pikachu, because why not? I mean, they're both yellow. Tails is into things technical and electronic. Pikachu's confirmed as an electric mouse. In Riders, Tails can do that thing he does with the plug to keep everyone nearby off his back. Pikachu keeps enemies off his with electricity of his own. Need I say more? They go pretty well together.
Amy here has a Ninetales, because she thinks it's cute, but let us all hope that she won't have to shave her. The reason for pairing her with Ninetales is that each time I see Amy, Ninetales comes to mind. This is especially true of the fact that when I see Classic Amy, Vulpix comes to mind.
And Finally, Knuckles has a "battle-ready" Lucario. Both are into fighting and at most times, their role are both mainly supportive loners. However, when there are obstacles where speed proves no help and flight becomes futile, that's when they bring in these powerhouses.