Sheep are a domestic animal added in SLASH'EM and ported to dNetHack. They are also always generated peaceful, making it trivial to tame them, but as their attacks are weak and base level is low so this is rarely worth it. Additionally they are herbivorous, and thus somewhat more difficult to keep fed than most pets. They are often found in Sokoban; be careful they don't get stuck between a boulder and a hole. Since they are always peaceful, they are easy to overlook.
A lamb is a baby sheep. It will grow up into a sheep at level 3.