Sega recently sent out a survey polling their fans on what franchises they would like to see crossed over with Sonic the Hedgehog. Joining an army of other pop-culture icons like Pixar and Capcom, My Little Pony was included in the mix. This is primarily centered around mobile games, though they did mention other platforms deeper in.
Will we see a Sonic X Rainbow Dash combo in the future? I'd be surprised. Beyond surprised. It wouldn't be the first time Sonic went absolutely crazy though. I'll be over here waiting for my Kingdom Hearts pony crossover.
More non-pony details at Gamespot.
Thanks to Slashx for the heads up!