There are many rules related to the Red Heifer in order to make it qualify for its special task. It is the vehicle by which people are purified for having come in contact with the dead.
Ideally, it should be three or four years old. It must not have been used to carry a load. It cannot get pregnant. It is allowed to have a cover on it to protect it from flies. It is mixed together with cedar branches and hyssop. It is taken out of the city, where it is burned. The remaining ashes are mixed with water. One drop on the forehead, does the purification. This is done twice over a seven day period.
Rabbeinu Bechaye lists the nine פרה אדומה’s that were used for this service. Moshe Rabbeinu offered the first. The second was attributed to Ezra. The third and fourth were connected to the High Priest, Shimon Hatzzadik. The fifth and sixth to another כהן גדול known as יוחנן in the second Temple. The seventh, eighth, and ninth, were burned by three lesser known names, אליהועיני, חננאל המצרי, and ישמעאל בן פאבי.
The tenth פרה אדומה will be offered by מלך המשיח. May he come speedily. The Red Heifer is the most difficult to understand and is the most intriguing Mitzva in the entire Torah.