In Bereshit 22 the ram appears in one sentence and is sacrificed the next. One could dream up plenty of questions about it (eg did the ram belong to anyone? did Abraham assume that it had appeared expressly to be sacrificed? is there a difference between sacrificing an unowned ram and one of one's own? did this act inaugerate ram sacrifice, or was it already an known ritual?) Are there answers to any of them?
The Mishna in Avot (chapter 5) lists the Ram as being created especially for being sacrificed by Abraham, and so it didn't belong to anyone. This opinion is also brought by Rashi on Gen. 22:13.
Radak, on the other hand, says that it probably belonged to someone and that, if Abraham would have found that man, he would pay him for the ram, since you can't sacrifice someone else's things. Chezkuni and Sforno don't accept this and say that, if the ram belonged to someone else, Abraham wouldn't have sacrificed it in the first place!
The Alshiche adds that the Mishna tells us that the ram was created during the last day of creation (as opposed to being created only when it was actually needed) to tell us that it was a perfect ram, better than any that could be born naturally.
It should also be noted that the Midrash (ילקוט שמעוני וירא צט) says that sacrificing that ram was the reason for Korbanot HaTamid.
All Kosher animals were sacrificed beforehand. Rashi mentions in נח that the reason the Kosher animals were 7 and not 2 was so that נח should bring a Korban from them.
Avraham obviously had to be Koneh the ram (from Hefker) because a Korban has to be your own. Otherwise it is not an offering. אם ארעב לא אומר לך.