My thoughts on Hecarim
I am a Hecarim/Sejuani main. Still mastery 6 on both due to a plague of S- but we gettin' there boiz.
Anyways. Multiple indirect buffs have caused Hecarim to become overpowered in his current state.
Top lane Hecarim is currently pretty balanced, and I don't think it should see (noticable) nerfs. As a simple-to-play champion, Hecarim's winrate should sit slightly above 50%, as whilst there are minor intricacies with playing him, such as timing your Q/W to go off during your E animation (to proc Q + TLD if you have it with the bonus AD) but before the MS is consumed, they are not major, are simple to learn, and relatively intuitive.
Hecarim's early-game snowball potential has always been fairly high, however, with the buffs to ghost it is now almost guaranteed. The amount of AD that Ghost gives you from your warpath passive is incredibly valuable early on. As such, at these early-levels you straight-up out-stat other junglers, and your ganks are hard to avoid due to how fast you appear, and the knock-back distance on your E.
If looking to primarily nerf jungle Hecarim, rather than lane Hecarim, I believe the knock-back distance on the E would be the place to start. By not knocking people back as far, it becomes easier for them to escape ganks. Perhaps just revert that change altogether, and revert it to doing a flat knock-back.
If Riot decides that Hecarim altogether is too powerful please do not make changes that will affect his late-game. Hecarim falls off late game already, and transitions largely into a fairly ineffective tank. If the ADC gets ANY peel it becomes extremely difficult to kill them in teamfights once they complete lifesteal, and Zhonyas can cock-block him almost as hard as it does assassins. His role is degraded to a meaty engage tank, a role for which he is outclassed by other champions like Malphite.
The following changes are the ones I would look to for Hecarim: Warpath: 15 to 30% bonus MS as AD => 10% => 30% bonus MS as AD Devastating Charge: Knock Back Distance: 250 to 450 => 300 (revert)
The reason that I do NOT suggest nerfing Hecarim's jungle clear by reducing his Q damage to monsters, is that his clear isn't all that healthy anyway. Whilst his clear reasonably fast, the early sustain is very poor and he is heavily gated by mana. If you DO reduce the Q damage to minions, please lower the mana cost - however, this would be a revert to a change made when top-lane Hecarim was overpowered.
On a similar note, the nerfs to Teleport suck for top lane Hecarim, but I think they suck for most top laners so I don't feel like this has affected him dis-proportionally. I do feel like it is worse for him than others (since Teleport is one of the main reasons to play Hecarim top) but I feel that it is only something to consider when making changes that could affect top-lane Hecarim.
I want to note that Hecarim top lane has fallen out of favor so hard it isn't even a displayed role on champion.gg now.
So please take me seriously when I say that top-lane Hecarim does not need a nerf. Yes Hecarim is out of hand. Please target his jungle performance.
TL;DR: Nerf jungle, but don't nerf his Q damage since his early sustain is fairly poor. Don't nerf top-lane Hecarim. Don't nerf his late game.