the same age it was when it died
You cannot give a Pandora's Box to a horse who is older than 25 years.
It depends on what you are using the horse for, how old the horse is, and the breed of the animal.
Once your horse is 6 months old it can go in a box. At the bottom of the screen (the bottom left of the history) there is a box titled 'Box'. Within that box there is a link that says 'My mare in her box'/'My horse in his box'. Click on this and select how long you want to leave your horse in for.
Just go to the right of your horse profile on howrse and click on the box that said reproduction. Then you can choose to cover a mare, or to click the box next to it. You can also click on the box information, you will probably see A box that said family tree+ offspring. If you click in the box family tree, it will show your horse mother and father. If your horse is a divine, you can also see it's grandpa and grandma. If you click on the box offspring, it will show it son and daughter. But your horse or pony have to be 2 years and 6 months old before they can mate. That where you should find the breeding page.
You cannot remove a horse from the safe haven on Howrse. Once the horse has been put in there it cannot be retrieved. You can revive a horse that was put into 'Heaven' so long as it is under 25 years old and you use a Pandora's box.
Using a cardboard box might help.
there is no game of Old MacDonald had a horse
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You can tell how old a horse is by the grove on its last incisor's.
(If you are trying to pass an achievement level, it is best to do this when your horse is 6 months old.) Try giving your horse treats like apples, carrots, turnips, and a salt stone. Don't put your horse in it's box because it will loose morale.
It depends on the age of the horse. Until the horse is 6 months old, it can't be put into a EC. When it gets to be 6 months old and older, it must be put into an EC, or it will not be able to go to bed at night, go in the box or meadow, or be fed carrots. Honestly, it's better to put a howrse in an EC then out of one.
If the horse is over 25 years old and not immortal, the horse will start losing a bit of its health everyday, which is natural, you can prevent this by making him immortal, or using a black orchid. If the horse is under 25 years old, it is most likely because you are feeding him incorrect amounts of fodder, making it over/underweight. Follow the instructions and the warnings when feeding your horse.