Roach isn't the only famous horse to be found in Kingdom Come: Deliverance. If you travel to the horse stable in Neuhof, you will find Epona -- Link's horse from The Legend of Zelda series.
The last time you saw her was in the Breath of the Wild. And as you know, Epona can be called by playing Epona's Song on the flute. KC:D developers know this well, and they even included a little test during the conversation with the horse seller.
If you decide to buy Epona, her master will ask you if you can play the flute, which is the reference to Epona's Song. Of course, you can't do that in Kingdom Come, so you will have to pay extra for Epona. But it's the thought that counts, right?