Did a bear ambush him?
Rav Sadya Gaon: He ambushed me like a bear or lion.
Rashi: Hashem switched to be like a bear in ambush.
Ibn Ezra: A bear come upon him from a path that he did not know.
Ri Kara: Even though the path is crooked, and a man cannot go on it to the straight path, if I will leave incarceration, He ambushes to tear me, like a bear ambushes to tear prey.
Palgei Mayim: He made the Yetzer ha'Ra so strong like a bear in ambush to make me sin.
What is the significance of "Ari b'Mistarim"?
Ibn Ezra: Some say that a bear announces (warns other animals) about a lion [nearby] 1 .
R. Avigdor: This refers to Aspanyainus Kaiser. Ari is written with a Hei (like Aryeh), to hint to Nebuchadnetzar, about whom it says "Aleha Arye mi'Subeko" (Yirmeyah 4:7), and he succeeded [to destroy the Mikdash] in the fifth month (Av), and a lion has five names - Ari, Shachal, Kefir, Lavi and Layish.
Palgei Mayim: After my measure [for sin] was full, he was like a hidden lion, who tears apart like splitting a kid in two. So Hashem brought calamity on me suddenly.
Ibn Ezra: This is difficult.