What are the lion and Re'em?
Rashi (14, 22): Just like You saved me from the Re'em (the Emori, "Asher k'Govah Arazim Gavho" - Amos 2:9), save me from the lion (Nebuchadnetzar).
Radak: Refer to 22:21:2:2.
Malbim: This is a metaphor. It is as if I was in the lion's mouth, and Hashem sent a Re'em to take him from there. It lifted him on its horns, and he is on the horns, with a congregation around him. He thanks for salvation from the lion, and requests salvation from the Re'em.
Why did he mention the Re'em's horns?
Radak: A Re'em gores with its horns - "Bahem Amim Yenagach" (Devarim 33:17).
Malbim: Refer to 22:22:1:3.
Did Hashem already save him from the Re'em?
Rashi: Yes. Refer to 22:22:1:1.
Radak, Malbim: No. Radak - "Anisani" is like Ta'aneni (answer me). This is common in Nevu'ah.