I think this should be changed to make them not be able to block instead of tapping them. Can't block feels more red than tapped.

Since tapping creatures in this fashion is almost identical to making them unable to block, and one is shorter than the other, I prefer tapping.

Hm. Personally I'd disagree with you - I think those tiny distinctions are fairly important for the colour pie. But Wizards clearly don't all agree because they print things like Thundermaw Hellkite, so I'm sure you could get away with this.

I think tapping things is tertiary for red. I could be wrong.

Also, this ability needs to be on one line, with Steam powered at the front.

Steam powered - Whenever Big Horn Thunderhoof attacks, you may pay for any number of target creatures. If you do, tap those creatures. If only colorless mana was spent this way, Big Horn Thunderhoof deals 2 damage to each of those creatures.

I agree small differences are important, I'm just in the camp that believes red should get tapping.

Wouldn't it make more sense to reverse the abilities? Most colors can tap, or cause not to block, but only red deals damage consistantly. I would think that this card would make more sense if it dealt two damage regardless, then tapped those creatures if you didn't pay red. Makes more flavor sense, too.

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Well the base card is red, so you can't cast this in the first place without red.

It's a valid idea to switch the two abilities around, I'm just not sure if it's all that better.

Tapping reminds me of Thundermare. Thundermare is awesome. Therefore, I like leaving this as it is.