When you catch a wild horse it must be put into quarantine for one week. Failure to do so will result in the possibility of infecting your other horses with a contageous disease, which could be fatal to them. Also if you board your wild horse at one of HE's public stables then you put all the other members horses that also board there, at risk of becoming sick. You need quarantine clearance before you can board your wild horses.

Quarantine Fees
One week stay is 200HD
Haay Eaters
Put your horse in quarantine
.