Is there any way to track how far you are on this?  I swear I've had the same horse for more than 20 missions and haven't gotten anything.  I wonder if the missions where you have to ride on a coach or train reset your count?

I kept the horse I got from Bonnie and got it right before I unlocked Mexico.
 
Did all the Stagecoach missions and it didn't reset as far as I know.

I've used the same horse I got from Bonnie and Ive still not gotten it, and I'm well into Mexico, even on missions where they provide a horse, I call in my own and I've still not gotten it.

I'm in Blackwater with the same horse and I've gotten it.

I got it right after the Fort mercer mission 

I had terrible luck with keeping horses. Always end up running them off cliffs or some shit. 

I got it after one of the missions in Mexico where I escorted this lady's sister to a boat to get her out of harm's way(trying to avoid spoilers)

Wait, there's an achievement for using one horse for a really long time? I've kind of just been taking whatever horse I want, whenever I want...

I started to try and get it after I completed the mission with Bonnie's father where you break the horse at the end, 
I got it after completing the mission before Fort Mercer

I used the same golden horse from the start, until I finished the game. I never got it.

Also, what constitutes a mission? It's pretty obvious all the story missions (the letters on the map) are missions, but do bounties, NPC/random encounters count as well?

I didn't find out about the achivement until I had 100%, doing another 20 missions just for a bronze trophey is too much effort. I can understand secret achievements but if it's going to be impossible to complete once you've gone past a certain point, don't make it hidden...

" Is there any way to track how far you are on this?  I swear I've had the same horse for more than 20 missions and haven't gotten anything.  I wonder if the missions where you have to ride on a coach or train reset your count? "

" I started to try and get it after I completed the mission with Bonnie's father where you break the horse at the end, I got it after completing the mission before Fort Mercer "

I had a horse for a long while but I was checking out a camp fire which was one of the trap ones and the first shot the guy fired was straight to my horses head :( , the next horse I had then fell of a cliff and since then they have been dying time from time.

At first I thought I wasn't gonna get this achievement after I completed what must have been 18 story missions and subsequently rode my horse off a cliff but alas I forgot about it andthen when I least expected it, many hours later I got it.

50 hours in and I'm still using the horse you tame with Bonnie, it's such a damn good horse.

got it after one of the seth missions, start a new game and be sure to do all the bonnie missions first, never hitch your horse unless its part of the mission, and watch out for ninja cougars

Damn cougars mauled two of my horses. Didn't get this one until the last area of the game. was really unsure whether I'd get it or not. 
 
I hate cougars.

Yeah, I'm going to have to start a new game in order to get this one.  Pisses me off that it's listed as a secret achievement.

can you get it by redoing missions from the stats menu?

So your horse can never die? Even if you whistle for the same exact horse after the other one dies?

" Yeah, I'm going to have to start a new game in order to get this one.  Pisses me off that it's listed as a secret achievement. "

I don't know if my game is glitched and I don't want to play another 12 missions to find out I have to start over yet again. 
 
I was using the kentucky saddler from the horse breaking mission for a few missions then decided to try for the $5000 bounty achievement since I just came across a pardon letter. So I kill tons of fools and get like a $3000+ bounty and my honor goes from positive to the bottom. I then start one of the Marshall's last missions and all horses are the regular plain brown one... even the one that has my logo when I whistle. But as soon as I mounted, they textures all changed to what they should have been and my horse was once again yellow. After the mission I saved at a campfire and when I came back I had a horse that I'd never seen before (and I already have 100%).
 
The horse that came was solid black with white face paint and had a bunch of red scars and cuts all over its body. It looked badass and made me think I somehow got the Warhorse texture for my Kentucky Saddler even though I don't have that DLC. So I decided to hitch it, and it actually saved. I went to start the first West Dickens mission and my horse was the yellow kentucky saddler again.
 
Anybody experience this? Should I keep on truckin for that achievement or should I start fresh again?

" @jakob187 said:

" Yeah, I'm going to have to start a new game in order to get this one.  Pisses me off that it's listed as a secret achievement. "

Yeah, same here. Does anyone know if your horse dies if that screws up the count? Even if you own the deed to that horse and have to call it again does it reset the count because your first horse died? Even though you have the same exact horse that you call if it happened to die? "

Ah, all right. Thanks, Shiro. That's what I figured.

One particular mission that messes up this achievement for a lot of people is "Exhuming and Other Fine Hobbies" with Seth.  During this mission, Seth asks you to steal one of the horses outside the jail holding Moses, to distract the deputies, so he can sneak into jail and interrogate Moses.
 
Here's the tricky part. If you simply jump off your horse and steal one of the deputies' horses, that will affect your ability to get this achievement. The safe way to complete this mission is to ride up to one of the deputies' horses, lasso it, and then drag the horse away to trigger the Wanted Level. Then simply drop the lasso and escape the Wanted search on your own horse. This allows you to complete the mission, and still maintain your string of missions without changing your horse at a hitching post.
 
One final note. I got this achievement on a quick second playthrough, and it unlocked after my 21st mission in the new game save was completed. If you skip all the cutscenes, race through the story missions, and don't do any side missions, it just takes a few hours to get the achievement.

Well I got it bitches!!!!!

Got it completely by chance towards the end of the game - never thought I would!

I kept wandering off and getting mine mauled by cougars, yet I still got it well before twenty missions with the replacement. I hear it unlocks sooner or later than it should sometimes. 

I'm going to try again but I started a brand new game yesterday and played for about 2 hours or so.  My mission counter said 21 the last time I looked at it but I did a mission or two after that (I had just landed in Mexico) and I had used the same horse the entire time.  Didn't get the achievement but I'm damn sure my horse never died and was using the same one the entire time (my war horse, he's easy to recognize).  Though, I suppose maybe I parked him somewhere and maybe he got randomly killed by a cougar or something?  I guess that could have happened if he was out of my sight.

Kind of pisses me off that I will have to play through the game again to get the achievement. Which is BS, since I've used the same type of horse throughout the game, but didn't realize until I was halfway through that the horse cannot die. Meaning every single fucking time my horse died from a fall meant I lose the achievement.

Got it the moment I got off the raft on Mexico. Just as "Far Away" was starting.

I'm positive that my horse died many, many times and I still got this achievement. Don't use a new horse deed from the general store or hitch a different horse to a hitching post and you should be totally fine. Just stick with the one you get from Bonnie and it's perfectly good for the entire game.

" I kept the horse I got from Bonnie and got it right before I unlocked Mexico.  Did all the Stagecoach missions and it didn't reset as far as I know. "

They took away the Kentucky Saddler I had from Bonnie, so I've been kind of fucked since then.

Ive been going through (15 missions in atm) and still have the horse from Bonnies breaking mission 
 
However i just started the West Dickens mission in which i have to do a cart race- any idea whether this is gonna screw my count up or anything>

I also used the horse i got from Bonnie and got this after one of the Irish missions (before Mexico). I haven't used anything other then my horse for traveling, and haven't used the fast travel or stagecoaches.

same as everyone, used the kentuky i got from bonnie and got it around half way thought mexico