Here are a few quick tips on how to place identification on your horse in an emergency.
Use a permanent marker to write your phone number on their hoof.
A dog tag makes for a great halter label. You can also braid it into their mane if your halter isn't readily available.
Microchip! Micro chipping will help Animal Services I.D your horse should it end up at the shelter.
Leather halters with name plates/phone numbers. Place a leather halter on your horse if you suspect that an evacuation may be eminent. Leather halters tend to break when caught on things like fence posts unlike nylon halters.