This is looking great! With all things that I end up liking, I want to share the few improvements I have. I know this game literally just came out so treat this as a giant list of criteria to tackle at whatever pace you want; I don't want to come off as demanding and/or pressure you into burnout.
HORSES
For the current horses, the only thing that I don't like is that Bullet'n Board has somehow lost his frog toes and he said he wants them back. Also Jovial transgressed back into a short dude when he used to be the tall boi and he don't appreciate it.
Of course, adding the nightmare horses would be cool (unless someone gets around to making them before you do).
Also, if possible, can there be a way to only show the horse's true name once they've won a race? Or at least make it toggleable from the start. This would of course need a way for the custom horse creator to add both names.
Lastly, how did you make the horse designs? I used Pixilart but I was wondering if you had any better suggestions? The only one I've made so far is called Dark Brigade (like the opposite of Light Brigade) and it's the template horse with an eye missing and RGB value 50-30-50
MAPS
For inbuilt maps, please get rid of the second-to-bottom tree in Hidden Forest I BEG. In all my testing it takes 50 years for a horse to get in that bottom section, and then most cases they leave soon after.
Since map making doesn't require artistic talent, I've started going back and making all the "canon" maps of HRT, and one thing to include if possible is different map sizes? The ones I've made so far work fine but somehow they feel a bit squashed?
Also PLEASE make it so that whatever map you have running saves after a race has ended. The amount of times I've forgotten to save and the map just completely reset without me saving it is embarrassing.
As for new maze themes, the vibrant red-orange-dark green scheme looks cool and a more "otherworldly" look for the Horror Horses. Maybe also classic space, water and desert themes.
OTHER
Firstly, a list of stuff I'd like to be able to customise/add on own stuff: Race Prize, Music, Ending Images, Ending Message, Decorations (maybe have decorations be shared between themes?).
A bouncing title like in the canon would be cool!
An option to randomly select horses to appear in a race, or at least randomly assign starting positions.
A key and gate pair, where one of the horses needs to collect the key in order for the gate to open; this will be open to all horses. Different variants could be a humble garden gate and key, a DANGER sign and hammer, a vault door passcode, and a door.... and a door knob.
LASTLY LASTLY, a way to bet on horses. I've made this vague on purpose since I don't have a specific ideal solution, but a way to keep track of your "winnings" would be engaging, especially with friends! Tying on to this, two records of horse victories; one for winning in that specific map and one total winnings (that resets when the game closes).
Sorry for all that, but again this is looking great so far and I await updates with great anticipation!
EDIT: Also what about an in-built way to record rounds? Also a way to speed up/slow down a round while it's happening.