The Ringwraiths, however, aren't riding Nahar.  Or Ancalagon.  They're riding, again, well-broken but perfectly natural horses.

Who, perfectly naturally, respond to predators by running away.  Fortunately for them, that's exactly what their riders are telling them to do.

Unfortunately for them, they've never before seen a predator this fast.

But the Ringwraiths were once kings, of many places scattered across Middle-Earth.

This one was from Far Harad, and he's seen cheetahs before.

It takes him a few moments to look back and see that the strange Man (another Istar?  A sorcerer who's somehow not working for their Lord?) has summoned a cheetah.  Who will, plausibly, be fighting on his side?  By that time, it's starting to jump into its run -

"Scatter!" he hisses in the Black Speech, and instantly turns off the Road to the north into the shadows of sunset.  One of them might still survive and stay horsed.

The other one, half a round later, turns off to the south with a wordless cry to put fear in beasts and persons alike.

Being a summoned creature doesn't make you immune to fear. 

But only one of its prey is making scary noises, and it's riding away from the other one. The cheetah leaps on the prey that tried to run north.

The horses left to their own devices would have done better; herd animals know to stick together in the face of the predator. Scattering is leaving the weak to perish.

Meanwhile, Gord advances on the nearer wraith and its confused horse. Can he kill the horse without being attacked by the wraith, and will this wraith also flee if he does?

The wraith waits for Gord to come close - and then jumps off the horse at him.  A knife, glistening in the twilight, is in his hand. 

...He'll back away, and do his best to evade or parry the knife! Who knows what thing can do. If the wraith insists on following him, he'll channel again.

And if the wraith gives him some space instead, he'll cast the Spiritual Weapon he's been holding in reserve, have it kill the horse, and then bring it back to hack at the wraith. If the wraith can't dispel it and doesn't have spell resistance, it'll do more damage than a single channel anyway.

Gord still wants to see if killing the horse actually makes any difference to the wraith's behavior. Strider said they borrow the horses' senses, but a wraith accurately blocked Gord's first strike when he was invisible and you can't do that with hearing and scent.