It is possible that is achievement is missable (I need confirmation). It is also very buggy and prone to glitching so people cannot finish it.I would recommend doing them before the point of no return.
Since it appears some races may not be available after endgame. Some people experience the Velen races disappearing after the isle for example, others have said a patch fixed it so it is clearly still broken and may not remain in your quests so better safe than sorry.
There are 11 possible races for you to complete in the game. I've included a video by Powerpyx (all credit to them for the video) showing the locations of each to help you all out as well. I believe you can simply go to the areas in the video and do the races (I did for several and it was fine), however, you can also pick them up as missions from notice boards throughout the game.1-3) Velen - Crows Perch:
You get to pick which order you race the people in.4-6) Novigrad - Vegelbud Residence
You don't get to pick your opponents this time AND
you have to have done the mission "Get Junior"
before you are able to unlock these races. Thanks to thomaskorat for clarifying that for me.Race 7 is obtained after Race 6 is complete.
It is also the one I believe makes this achievement potentially missable. I recommend saving after race 6 just to be sure
. After completing race 6 (start to leave the area) I was approached by someone and given a message. You must read the message from Carlo Varese (cleaver) and visit him in Novigrad in order to take part in this race. I believe it is possible to fail this race (I am sure I did the first time so Reloaded and replayed to ensure I won) so a save prior to doing it is a good idea. I've also heard reports of this mission glitching during so people can't finish it.
Failing the race nulls your achievement so save prior!8) Skellige - Rannvaig9) Skellige - Fayrlund10) Skellige - Blandare/Palisade11) Final race is in Skellige - Larvik
Race 11 SHOULD unlock after all the other races are done. However, for me and other people it has glitched where the person you talk to in order to start the race isn't showing up making it impossible to start and finish the race. I have no fixes for this as of yet as my personal reloading to an earlier save didn't work. They may patch it in the future or you may just have to get it on a second playthrough. Edit:
This should potentially be fixed with patch 1.07
As others have mentioned the tips for horse racing usually involve sprinting and trying to position yourself in front of your opponent to prevent them passing when you need to recharge stamina. Luckily most races are repeatable to give you several tries. Completing Races also gives you better gear. Including more inventory carry weight, more horse stamina and horse blinkers to help with horse fear. So you can save your money and not buy horse gear from merchants. It can also make Races much easier (especially the extra stamina).Edit:
NicholiGinavaef mentioned in the comments where to find the Mastercrafted Cavalry Saddle with 75 stamina:"in the Smuggler's Cache north of Kaer Almhult. Kear Almhult is the small island west of Eldberg Lighthouse. The Lighthouse is north of the village Arinbjorn on the western side of the main island in Skellige. This saddle is not one of the race prizes that gets handed out. A lot of stamina really make these races easy."
If you vote negative could you let me know why so I can improve this solution because as of now I'm not sure how much is accurate and how much is the game having a few bugs.
A video showing locations by PowerPyx for a visual guide if you don't have the notice board quests and wish to find them.