might be a bug or a feature, so after fighting Kalysea (horse boss) and getting out of the map/leaving the map you can go right back and fight them again, it is a good way to farm items. though you could make it so you can only fight them a certain amount of times then have it so the player has to lets say fight some waves or the other boss then the first boss gets reset so you can fight them again
You could also make the boss fight become increasingly difficult the more times you try to do it or only let her drop items the first time. Currently I feel that Kalysea is too easy to fight, even on the hardest difficulty without items you can still win by using the fence as a barrier. Just keep walking in circles around it while she is chasing you and she'll never be able to catch you, especially if you also use spells. I think it would be a good idea to try to make this strategy less OP, a simple fix could be to put mud around the fences or make the grass slow you down (or arouse you, even), although I think this strategy could still be abused then. Something to think about I guess.