From this page of the Valheim game guide, you will learn how to defeat the Deathsquito.
It is a hostile entity that lives in the Plains. It is a small-sized creature which can be easily overlooked. Additionally, it often flies up from behind. The easiest way to locate it is to listen to your surroundings and catch on the buzzing.
This type of enemy can do a lot of damage - especially if you have an armor weaker than silver. On the other hand, the Deathsquito has very few health points, so its only advantage is its small size and speed. To eliminate it, it is best to use a bow. Carefully follow its trajectory and wait for it to attack. At this point it will be flying straight on towards you, which makes it easier to shoot.