This week's פרשה features the epic battle between the short-handed four kings, אמרפל, אריוך, כדרלעמר and תדעל, and the five kings, ברע, ברשע, שנאב, שמאבר and... wait, was the name of the fifth king? When the five kings are mentioned, the last is "ומלך בלע היא צער". Rashi explains that the city of בלע was also known as צער. The פסוק could not be naming צער as the king of בלע because of the feminine "היא." If צער were the name of the king of בלע, it would have read " ומלך בלע הוא צער." So what was his name and why is it left out?
A number of answers are suggested. רמב"ן states that בלע was a small city and so the name of its king was left anonymous due to his relative insignificance. שערי אהרן points out that the names of the four other kings are apparently nicknames alluding to each one's wickedness as רש"י thoroughly explains. From the story of the destruction of סדום in next week's פרשה we learn that צער was the least wicked of the five wicked cities slated for destruction. Thus, the king's name is left out due to his relatively insignificant wickedness.
Suprisingly, however, חומת אנך and ספר הישר actually write that the name of the king was בלע. I am not sure how the grammar of the פסוק works and why this king is differently introduced than the others but this is the only offering I have found as to the actual name of the king.