Coleco Swat Swat the Mosquito (1979, LED, 9 Volt, Model# 2035)
Not actually a handheld game, looks like fun though... :) The Mosquitos are represented by lights flashing all around, when one lands in your white circle, your have to swat him fast with the little fly-swatter. Finally found in a beat-up box... Proves it exists though! :)
Do I have this? Yes, boxed (bottom pics are mine).