ANSWER: You have asked some interesting questions about the 'lost sheep' parable (also referred to as the 'good shepherd') in Matthew 18. It is one of at least seventy parables the Lord used in his teachings and the 26th one listed in Matthew. Although the good shepherd is also found in Luke 15, we will study Christ's words found in the first New Testament gospel.
What do you think? If a man (a "good shepherd") has a hundred sheep and one of them has gone astray, does he not leave the ninety-nine . . . And if he finds it, truly I say to you, he rejoices over it more than over the ninety-nine that did not go astray (Matthew 18:12 - 13, HBFV).
Jesus opens his parable by asking what should a good shepherd do if one of his many animals in his flock is lost (Matthew 18:12). He answers his own question by stating the keeper of the flock would leave the 99 sheep that he has and pursue the one that strayed. When the sheep is found the shepherd is said to be happier at finding it than the fact that the rest of the flock was not lost (verse 13).
The main characters of this story are used only as an analogy that tie human emotions and logic to the emotions and logic of God to help us better understand how He thinks.
It should be obvious that the parable does not present a real life situation. In the real world, one would want to know the details of how ninety-nine animals were to be protected while the shepherd was gone. Adding such detail to the story, however, would serve no tangible purpose.
Even today, the herdsman would call a friend or family member to watch over the flock if he expected to be gone for an extended period. Or perhaps the good shepherd knows the area where the sheep are grazing to be safe for the few hours he would be gone. If the herd would be in danger, he probably would not leave all of them to search for just one. These mundane details, however, would do nothing to teach the lesson intended.
A non-Biblical example of a parable is the story told of a race between a tortoise and a hare (rabbit) as part of Aesop's Fables. On the surface, this story in not a reflection of what happens in the real world, as hares (rabbits) and tortoises not only do not speak to each other, they do not compete against each other in races. The purpose of the story, however, is to teach the lesson that steadfast progress and good hard work sometimes are better than raw speed in achieving a goal.
The purpose of the Good Shepherd or Lost Sheep parable is stated in verse 14 of Matthew 18. God, because His existence is the essence of perfect love, desires all those whom he has called to enter into his kingdom. The 'little ones' he is primarily referring to are those people who are new believers in Christ.