The Varasyo Or The Sacred Bull
A white bull known as the Varasyo is brought into the fire temple where the nirangdin ceremony is to be performed. A single black hair on the body of the bull disqualifies it from being used as a sacred bull. The word 'Varasyo" comes from the Avesta word 'vareca' meaning hair, because the hair of this while bull is used symbolically in the Yasna ceremony. A metallic ring used in the ritual is known as 'varas ni viti' i.e. the ring with the hair. The hair of this sacred bull is put round the ring.
The Parsis has such white bulls in their principal towns. They are held useful for two purposes.
1) is as described above for their urine, which, together with that of other ordinary bulls is consecrated.
2) is the use of their varesa i.e. hair which is used in the Yasna to serve as a kind of hair seive. This use is referred to in the Visparad(Karda X, 2 varasai Haomo angharezanai. i.e. the hair to pass, as through a seive, the Haoma juice) This bull is not used for domestic purposes.
On its death, all the liturgical services, wherein the varesa or hair is used, are stopped in the town or towns. Another white bull is immediately sought out and consecrated. Until it is consecrated, all the necessary Vendidad,Yasna and Visparad ceremonies in which its hair is used cease to be performed in the town, and are directed to be performed in other towns which have their separate bulls.
Source - Global Directory of Zoroastrian Fire Temples by Marzban J. Giara