Who was Balak?
Rashi (on Pasuk 4), Ramban #2 (citing a Midrash) and Targum Yonasan (in Bamidbar 31:8): He was one of the five princes of Midyan, and when Sichon was killed, Mo'av appointed him king (to defend them against Yisrael).
Ramban #1 (on Pasuk 4) and Seforno: He was a renowned warrior of that time. 1
What exactly, did Balak see that made him afraid?
Rashi and Seforno: He saw how the two kings upon whom they had placed their trust were unable to stand up to Yisrael, and he figured that that being the case, they would certainly not be able to.
Ramban: Because Yisrael was a far more numerous nation than Mo'av, 1 a. since the latter was a relatively new nation, and b. because he heard about the ongoing miracles that Hashem had performed with them and with their forefathers.
Seforno: He saw how a. when Sichon refused to let they Yisrael pass through his land, they annihilated him, and b. how their victory did not come about in a natural way. 2