Aqua Tail or Outrage for Rhyperior?

Question

This is in Platinum.

Answer 1

Neither

Neither is recommended. Outrage has high base power, but provides almost no coverage whatsoever, and traps you for 2~3 turns.
Aqua Tail will rarely come in handy, because you already have Earthquake (or at least you should) to cover Rock types, and Stone Edge is also quite mandatory so Moltres is covered.
If you had to choose one though, I would say Aqua Tail, because it can be useful against other Ryperior.

Recommended moves

• Ice Punch - It's a generally good move that covers Grass, and is a great move to keep Gligar and Virizion, two common Rhyperior switch-ins, from completely walling it.
  If you're using it for OU or you're not playing Showdown, it can used effectively to deal severe damage to Gliscor, and provides coverage against a Dragons, which are common in OU.

• Thunder Punch - Provides coverage against Water types, if you can out speed them, but tbh doesn't do much else.

• Stealth Rock - The last slot doesn't have to be an offensive move, because QuakEdge and Mega Horn provide good type coverage already.
  Stealth Rock is a useful move that can help your overall team, dealing 1/4 damage to Fire, Flying and Bug types, and can shutdown Talonflame completely if you're playing OU.

Comments

Any other, better move?

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Please give me the rest of your moveset, then I will probably be able to recommend one.

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Rhyperior@Focus Sash
-Earthquake
-Stone-Edge
-Megahorn

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I recommend Ice Punch, it provides good coverage, or Poison Jab is also an option to hit Fairies and any Grass types you can outspeed.

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not ice punch, because its power is the same as earthquakes power on for example groundtypes, and ice punch, and not poison jab either, since theres no fairies in pokemon Platinum. Sorry i didnt tell you before.

Answer 2

I wouldn't say Outrage, reason being is that you're only going to use it when facing Dragon-types; an Ice-type move will irrefutably do a better job regarding coverage. If you absolutely needed a Dragon-type move, Dragon Rush is the way to go in Rhyperior's circumstances, although it seems you would have to start again. Bearing THAT in mind, I'd then go for Dragon Tail.

Moving onto Aqua Tail; it's not too shabby. But Stone Edge and Earthquake will cover the types you're planning to use it on - in light of this, an elemental punch would be nice, e.g. ThunderPunch - it should function well when facing Water-types that plan to use special attacks that will take advantage of your low Special Defense and as you're aware, there are a plethora of Water-types out there; it's something worth thinking about. Fire Punch is a close second (regarding Grass and Ice-types), but I personally feel ThunderPunch is the most beneficial of the elemental punches, due to the fact your Megahorn can cover your Grass weakness. Shielding yourself whilst synchronously taking normally resistant Pokemon out would be lucrative on your part.

But yeah, in sum, I wouldn't recommend any of the moves you've listed, but more an elemental punch.