I'm actually wondering in the trailer we do see that we can get a Houndoom in the Grand Underground. So what fire Pokemon would be better Rapidash or Houndoom?
Houndoom by a pretty considerable margin. That being said, I've seen a Diamond-version Nuzlocke with just a Chimchar and Ponyta and Ponyta managed to beat Wake's Gyarados with the move Natural Gift and a Pecha Berry to turn it into an Electric-type move. That run was mostly insane though..
But Houndoom is the better Fire-type and also gives you a great Dark-type while you're at it. Also, Houndour evolves at level 24. Which means you get a BIG upgrade to your team basically as soon as you're catching it.
I still find it excruciating to train a Ponyta up to level 40 though..
But Rapidash has received a lot of fun toys since GenIV (Poison Jab, Wild Charge, PlayRough, SmartStrike)..
I'm definitely going to try to play the main story without a single Fairy-type on my team, because I don't want to risk Cynthia being easy.
I don't know if ILCA and Masuda have accommodated for the type, so I'm not taking any chances. I intend for my battle against Cynthia to be a memorable one.
If possible I'm intending to use the Togepi-line. But knowing the D/P-games, you could only get the Shiny Stone at Iron Island (and the Battle Zone) and in Platinum you could also get one at Route 10, but you needed Rock Climb. What this means is that the game essentially gatekeeps you from using Togekiss until at least the sixth gym..
By which point you're really only facing Steel, Ice and Electric Gyms.. Which Togekiss is not well-suited for.
The Galactic admins and Cyrus are predominantly Poison-type users (and the occasional Bronzor+Glameow iirc). Togekiss really only has an advantage against Cyrus' Honchkrow (and Weavile, but that's a faster Ice-type) and Saturn's Toxicroak, but trades off the Flying vs. the Poison match-ups.
Meaning. While Togekiss as a Fairy-type in Sinnoh might seem broken.. It will struggle during most of the latter portion of the game, until you hit Aaron/Cynthia.
So I intend on using it -if we can get Togepi early-, but not forcibly train it up to match my other levels. An underlevelled Togekiss v. Cynthia sounds like a fair deal.