There are two people in chumash that were named before they were born — Avraham was told that his son’s name was going to be Yitzchock and Hagar was told that her son was going to be named Yishmael.
I’ve always wondered if Hagar told Avram that detail of her encounter with the angel, or if she kept it to herself until after Yishmael’s birth to see if the word of the angel would come true (which it did).
Look at the Rav Sapurna Breishis (16,15) who says that the name Yishmoel was applicable because of both. Avraham davened for him whereas Hagar heard it from the Angel. It seems that Avraham called the name on his own because Hashem listened to his tefila Breishis (17,20).
As a side question, Rivkah was told, the older will serve the younger. Did Yaakov Avinu know this? The Maaseh Hashem explains the connection between Aisov asking to be served the lentil food and Yaakov Avinu wanting him to sell his birthrite. Yaakov Avinu said, if you, Aisov want me to serve you, I must be the older.
Wolf, does this answer your question?
The Ramban mentions both possibilities. I’ll look it up and post it.