Since the first RFID patents (#3,914,762) were issued in 1975, RFID tags have been attached to millions of animals. A number of countries have implemented mandatory tagging of domestic animals, such as New Zealand requiring all licensed dogs, with the exception of farm dogs, be tagged. Similarly, Northern Ireland and Israel have requirements for all dogs being tagged.
Source – Wikipedia
The most common RFID tag for animals is roughly the size of a grain of rice, and is inserted under the skin.
The majority of RFID tags in use today operate in the 124.2 KHz to 135 KHz frequency range. As a result of this low frequency use, the range is limited, with typical read ranges less than thirty feet. Most of the tags will have from 112 bits to 128 bits, depending on the technology adopted. Besides the unique identification number of the tag, a number of bits are reserved to provide the country code as identified by ISO.
The tags can be inserted under the skin or may be clamped on the ear along with other more visible tags for animal identification.
Source – Wikipedia
There have also been major efforts by veterinarians to have household pets tagged. There are a number of national organizations that act as a clearing house for pet recovery based on an animal being tagged.
While the majority of the tags used in animal tracking applications operate at the low frequencies noted, Amtech Corporation did develop a longer range system.
Source – Wikipedia