This is an old bug, appeared to be in the game for several patches. Same problem will occur to boats, sometime placing boat and it disappear on your UI hotbar but did not actually placed in the world. Normally I will re-log into the game to fix this too. So did your friend doing the same to fix the problem for the horses.
Unfortunately that's all we can do for now until they figure out how to fix it.
N. Neo
This is an old bug, appeared to be in the game for several patches. Same problem will occur to boats, sometime placing boat and it disappear on your UI hotbar but did not actually placed in the world. Normally I will re-log into the game to fix this too. So did your friend doing the same to fix the problem for the horses.
Unfortunately that's all we can do for now until they figure out how to fix it.