What were the horns on the altar?
1. The Brazen Altar
These projections at the four corners of the altar of burnt offering were of one piece with the altar, and were made of acacia wood overlaid with brass (Exodus 27:2 , "bronze"). In Ezekiel's altar-specifications their position is described as being on a level with the altar hearth (Ezekiel 43:15 ). Fugitives seeking asylum might cling to the horns of the altar, as did Adonijah (1 Kings 1:50 ), which is one proof among many that worshippers had at all times access to the neighborhood of the altar. On certain occasions, as at the consecration of Aaron and his sons (Exodus 29:12 ), and a sin offering for one of the people of the land (Leviticus 4:30 ), the horns were touched with sacrificial blood.
2. The Golden Altar
The altar of incense, standing in the outer chamber of the sanctuary, had also four horns, which were covered with gold (Exodus 37:25 ). These were touched with blood in the case of a sin offering for a high priest, or for the whole congregation, if they had sinned unwittingly (Leviticus 4:7, Leviticus 4:18).
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).