The law requires all horses, ponies, donkeys, mules and zebras to have a passport and the correct ownership to be recorded within 30 days of any ownership transfer on the Governments Central Equine Database. This has been law since 2005 but since July 2009 the law has been updated by the EU (Commission Regulation EC 262/2015), with the details provided in the legislation shown below. Now all equines are required to also have a microchip linked to their passport. Passports issued before July 2009 may need updating.
There are some variations in the law depending on whether you live in England, Scotland, Wales or Northern Ireland. In each case you will need to read Commission Regulation EC 262/2015 as well as the legislation listed for your country.