Critics who claim that references to "sheep" in the Book of Mormon are anachronisms ignore important contradictory evidences.
It is likely that the Jaredites brought sheep with them to American, although they are not mentioned by name specifically (see Ether 6:4). They may have been among the “flocks” mentioned in Ether 9:18.
The apparent recovery of sheep wool from a pre-Columbian burial site near Puebla (southeast of Mexico City) indicates that sheep existed in that period.
Petroglyphs from Mexico and the southwestern United States depict many prehistoric drawings of sheep. These strongly support that the association of sheep and man occurred in America before this animal was brought over beginning in 1493 with Columbus’ second voyage.
Research indicates that sheep originally crossed to North America over the Bering land bridge from Siberia where at one point the population in North America peaked in the millions.
It is impossible to prove that sheep did not exist for several reasons. The exact creature being referred to is unknown due to the differences in languages. Also, excavations have uncovered only a small fraction of the information needed to identify the precise animals that lived anciently.