a red heifer:

The following curious particulars have been remarked in this ordinance:

1. A heifer was appointed for sacrifice, in opposition to the Egyptian superstition, which held these sacred, and worshipped their goddess Isis under this form; and this appears the more likely, because males only were chosen for sacrifice. So Herodotus says, they sacrifice males, both old and young; but it is not lawful for them to offer females.
2. It was to be a red heifer, because the Egyptians sacrificed red bulls to the evil demon Typhon.
3. It was to be without spot, having no mixture of any other colour. Plutarch says, the Egyptians "sacrifice red bulls, and select them with such scrupulous attention, that if the animal has a single black or white hair, they reckon it αθυτον, unfit to be sacrificed."
4. Without blemish. (See note on "And whosoeverª¹ offerethª° a sacrificeª of peace offeringsª unto the LORDª to accomplishª° [his] vow,ª or¹ a freewill offeringª in beevesª or¹ sheep,ª it shall be¹ perfectª to be accepted;ª there shall be¹ no¹¹ blemishª therein." {Lev 22:21}.)
5. On which never came yoke. Because an animal which had been used for a common purpose was deemed improper for sacrifice. "And the priestª shall takeª° cedarª wood,ª and hyssop,ª and scarlet,ªª and castª° [it] into¹ the midstª of the burningª of the heifer.ª" {Num 19:6} + "As for¹ the livingª bird,ª he shall takeª° it, and the cedarª wood,ª and the scarlet,ªª and the hyssop,ª and shall dipª° them and the livingª birdª in the bloodª of the birdª [that was] killedª° over¹ the runningª water:ª" {Lev 14:6} + "Come¹ now,²°¹ and let us reason together,ª° saithª° the LORD:ª though¹ your sinsª be¹ as scarlet,ª they shall be as whiteª° as snow;ª though¹ they be redª° like crimson,ª they shall be¹ as wool.ª" {Isa 1:18} + "Andª fromª Jesusª Christ,ª [who is] the¹ faithfulª witness,ª [and] the¹ first begottenª ofª the¹ dead,ª andª the¹ princeª of the¹ kingsª of the¹ earth.ª Unto him that lovedª° us,ª andª washedª° usª fromª ourª sinsª inª his own²¹ blood,ª" {Rev 1:5}