"If you own a herd, flock or even a single animal of the most common livestock - such as cattle, sheep, goats, deer, horses or pigs - there are rules that you must follow to stop the spread of diseases, particularly those caused by highly contagious viruses. The rules cover the identification, tracing and movement of livestock." (source: gov.uk)
As part of those rules all livestock need to be tagged and in some cases, for example cattle, also have a passport.
The tags are clipped onto the animal, usually the ear, and can be used by DEFRA to identify the animals owners.
Report stray animals
If you come across a stray animal or an animal is involved in traffic accidents or incidents please make a note of the identification number as this will allow the animals owner to be contacted.
All identification requests for livestock that have strayed or been involved in a traffic related accident or incident, if tagged, can go through the DEFRA Rural Services on: 0300 0200 301 (Animal and Plant Health Agency), select option two and then option four.