I tried to clarify all of that in my variant Journey Rules but one thing is sure; horses and ponies are quite different when it comes to navigating tight/steep/narrow/uneven terrain.

Whereas horses can be severely hindered by wild terrain (such as we can imagine Mirkwood to be), ponies (and donkeys for that matter) are smaller, lighter and far more sure-footed than their larger cousins. Whereas horses have advantage of speed and power, ponies are more apt to navigate rough terrain. They are also braver and not as easily freaked out, making them more manageable around heights, enclosed spaces etc. They have historically served as efficient beasts of burden in African and Amazonian forests, and mountains from the Andes to the Himalayas. I for one would allow ponies to reduce Fatigue increase / # of tests in Mirkwood.

Since TOR does not make any difference in speed between horses and ponies, we could as easily make the same abstraction about their sure-footed-ness (although I decided not to in my houseruled variant)

I agree with you. I don't know if it helps, but Tales from Wilderland specifically mention that the "pony rule" is applicable to a journey on the elf-path (although there's a mention of the old rule, not the revised one).

And thanks Fictionaut .