Mount the horse. Open the console. Type:

KillHorse()

Recall Roach. Save game. Fixed.

How do I get the console to work exactly?
What are the steps?

I tried installing the console mod here: http://www.nexusmods.com/witcher3/mods/1334/?tab=1&navtag=http%3A%2F%2Fwww.nexusmods.com%2Fwitcher3%2Fajax%2Fmoddescription%2F%3Fid%3D1334%26preview%3D&pUp=1
but it didn't work - I press TILDE ~ and nothing happens.

Thanks Hickory! Just got it to work.
I got it working by installing the Visual C++ Redistributable for Visual Studio 2015 x64 bit version.
Then mounting the horse (forgot to mount!) and hitting the console command, and saving.

Thanks for the help!
I hope this issue won't return.
Would you recommend leaving the Console Debugger mod installed? Or remove the DLL and folder from bin>64bit?

It shouldn't return. It's a leftover from a bug that has been patched, but remains in saved games. Now you have fixed it and saved -- that's why I said to save -- the proper information is now in your saved game.

Unless you intend to use the console a lot, I would remove it, definitely.

Cool. Good to know!
I'll remove it right away, but keep the files aside, just in case. Afterwards I'll perform a Verify Game just to make sure everything's fine.

You think I should uninstall the Visual C++ Red 2015 X64bit as well? Just wondering if it's excessive software (or bloatware)

Btw, do you know how to set GOG to send me Email Notification for the Community/Forums replies, such as yours? I checked the account settings and couldn't find it. I didn't get any email notifications for your replies, I had to manually check and re-check to see if you replied to my post, which is incontinent. Thanks.

The C++ files I would leave. There are many games and other software that require them -- they're official Microsoft files, so they're nothing untoward. The size is minimal, and they are inactive until required by a program. If you *really* don't want them, you can remove them from Control Panel, but should they be required again, you will have to reinstall.

I don't use Galaxy, but as I understand it, you don't get reply emails for forum activity, since the forum software doesn't tie in with Galaxy at all. There is no option to receive emails for forum replies without Galaxy, either; not part of the software, which is GOG's own.