Jamieson, Fausset and Brown on Isaiah 21:8

8. A lion--rather, "(The watchman) cried, I am as a lion"; so as is understood (Is 62:5; Psa 11:1). The point of comparison to "a lion" is in Re 10:3, the loudness of the cry. But here it is rather his vigilance. The lion's eyelids are short, so that, even when asleep, he seems to be on the watch, awake; hence he was painted on doors of temples as the symbol of watchfulness, guarding the place (Hor. Apollo) [Horsley].