There are different varieties of sheep. Probably the flocks of Abraham and Isaac were of the wild species found still in the mountain regions of Persia and Kurdistan.
After the Exodus, and as a result of intercourse with surrounding nations, other species were no doubt introduced into the herds of the people of Israel. They are frequently mentioned in Scripture.
The care of a shepherd over his flock is referred to as illustrating God’s care over his people (Psalms 23:1,2; 74:1; 77:20; Isaiah 40:11; 53:6; John 10:1-5, 7-16).
“The sheep of Palestine are longer in the head than ours, and have tails from 5 inches [12.7 centimeters] broad at the narrowest part to 15 inches [38.1 centimeters] at the widest, the weight being in proportion, and ranging generally from 10 to 14 lbs., but sometimes extending to 30 lbs. The tails are indeed huge masses of fat” (Geikie's Holy Land, etc.).
The tail was no doubt the “rump” so frequently referred to in the Levitical sacrifices (Exodus 29:22; Leviticus 3:9; 7:3; 9:19). Sheep-shearing was generally an occasion of great festivity (Genesis 31:19; 38:12, 13; 1 Samuel 25:4-8, 36; 2 Samuel 13:23-28).