Rashi on Exodus 15:15/רש"י על שמות ט״ו:ט״ו

1אלופי אדום אילי מואב [THEN WERE PERTURBED] THE CHIEFTAINS OF EDOM, THE MIGHTY MEN OF MOAB — But surely they had no cause to fear anything because they (the Israelites) were not marching against them! But the explanation is, that they were perturbed by annoyance, because they were annoyed and distressed by the glory that Israel had achieved (cf. Yalkut Shimoni on Torah 251:11).

2נמגו means THEY MELTED — just as, (Psalms 65:11) “Thou makest it soft (תמגגנה) with showers”. They said, “They are marching against us to destroy us and to take possession of our land” (therefore they melted away from fear, and were not merely perturbed in mind) (Mekhilta).