In the Jewish tradition, it was livestock a lot of the time: sheep, goats, fatted calves, that sort of thing. If you look through the Old Testament, especially Leviticus, it spells out what sacrifice or other method of atonement was required for each type of sin.
Hence the term "Lamb of God" as a name for Jesus Christ: the idea is that as a perfect sacrifice, He is infinitely more satisfactory to God than a regular mortal lamb would be, and hence is enough sacrifice to answer for all of humanity's sins ever.