Okay, first of all, you need to understand Donphan. Donphan can do 60 damage for 1 energy. That means that Donphan can start attacking 1st turn with a rare candy or BTS. This means that anything slow will not work against it, meaning Frost Rotom.(Not wash! look it up) Frost Rotom is by far the worst Donphan counter I can think of. 3 energy, 2 is you use DCE, and then you rely on his retreat cost. Flyphan, anyone? That means that Donphan's retreat cost is gone, and he can retreat to a better attacker. That also means you're screwed.
Besides, even if you get Frost Rotom out turn 1, with a Water energy and DCE, Donphan can still knock you out before that, or you knock his out, then he sends out a second Donphan Prime and you're screwed again.
Donphan prime is simply too fast for Frost Rotom.
Dewgong SV isn't a bad counter, and probably the best. 80 solid damage against Donphan is nothing to scoff at, and Thick Fat is a nice resistance against Chari and Kingdra decks. But, once again, Dewgong can be too slow. If you send out a Seel first turn, Donphan is there to knock it out. If you build up a Dewgong on the bench, then your opponent already has a full army of Donphans on his. Maybe even a Flygon, which can OHKO your Dewgong. The water energy is a pain to search for, especially in a fire deck when RR is getting rotated.
Any other counters are decks of themselves. Umbreon UD, Kingdra Prime, Gyarados, etc. You shouldn't need a donphan counter, if your using Houndoom as a tech or attacker. Easy solution, don't use Houndoom. Blaziken PT does the same thing ad is way better. Especially combined with Blaziken FB Lv.X. Think about it.
The best way to beat Donphan is to overpower him, or outspeed him. If you can do that, you're fine. Trust me, I know. I've played Donphan Prime since he came out. I know my stuff.
P.S: Decmaster, don't insist cards your not sure about. First, you said Wash Rotom, when it's actually Frost Rotom. Secondly, a topic like this arose a while back, and everyone agreed that Frost Rotom wasn't a good counter. Do the research next time.