There are two factors which we should consider while completing Horses bestiary entries:
1) Horse Station World Change
https://tibia.fandom.com/wiki/Horse_Station_World_Change
Possibility to complete this bestiary gives us an active world change called "Horse Station". We can notice two stages:
1) horses are in the Thaian Horse Station and we can rent them as a mount,
2) Horses are on the loose and spawn in the wild around the station.
First stage doesn't let us finish horses bestiary entries, however during the second stage we will meet all kind of horses: , , + additional rare entry: Wild Horse spawning wildly among the trees and around Troll camp to the east of Thais.
2) Spawn of the Wild Horse
During the second stage we can usually meet small amount of ordinary horses around Palomino's barn. To meet more of them we should come there in the time of Wild Horse spawn. Every three hours since the server save, there is a respawn of a big wave of horses which may contain from 0 to 3 Wild Horses.
Their respawn area is marked below.
I marked whole area east-south from Thais, because horses are quite fast and they split up very easily.
Worth to mention that, unfortunately, the big wave works only once. After we kill all horses that appeared with Wild Horses, the respawn will work as usual - slowly with way less monsters around.