The people we commonly call "sheep" today would be the goats in the analogy. Shepheard's always had both sheep and goats, and the heritage breeds of biblical times were nearly identical, not clearly defined animal breeds like we have today.

The difference was most evident in their behavior.

Sheep learn the voice of a single Shepheard, they cannot be fooled into following someone else. They will follow only their Shepheard, and they will go wherever the Shepheard leads them. Even the sound of his voice can calm them when they are afraid.

Goats on the other hand, could care less about the Shepheard's voice. Instead they will choose among themselves a leader, whichever goat is the most popular, and they will stubbornly follow that leader to their own destruction.

Separating the sheep and the goats was an easy matter. The Shepheard calls the sheep, and the goats stay behind with their "leader."

So when Jesus spoke of a Shepheard's voice and his sheep, and explicitly left out mention of the goats, people of the time knew exactly why the goats were not worth mentioning.