A horse being frozen can be a couple different things.     It could be that the person who owns the horse is currently on an account freeze (Account → Settings → Very Bottom of Page) or it could be the horse is in a frozen tab (Estate → Manage → Tabs).

When a horse is ‘Frozen’ its age up and cooldown timers are essentially on pause until the horse is no longer frozen :) (its out of commission for everything)   Its nice for when you have a project on hold, can't show / career everyone, have limited to no time to be on for a day, maybe you are going on vacation for a week, etc.