High Kingdom's Splint Boot is an item which transforms your chosen horse into Zara Tindall (nee Phillips)'s champion horse High Kingdom. It is no longer in the Black Market but you can still get horses with it or get it through exchanges. It is non-compatible with any other customisation items. It gave the horse the following bonuses:
Stamina: +25
Speed: +20
Dressage: +25
Gallop: +21
Trot: +22
Jumping: +25

Like the Shutterfly Boot on the int?
But why is it showing in item exchanges? Is it an item that is still being released?
Just seems weird of that's not the case

I mean released in limited capacity as its not currently available.

It was a BMI introduced for a limited time on the UK server when Zara temporarily partnered with Howrse in 2014, and the item hasn’t been made available again since the partnership ended. I can’t remember how the BMI was given out, only that it was available for free and I ended up with multiple in my inventory despite having no passes to spend as I was a new player back then.

At the time it was made available to trade in the item exchange, and that feature was never removed as some older players today still have the item in their inventory.

I doubt it will come back because it’s an item based on a real person and horse who no longer have any involvement with the game.

Thank you <3