Louse

Lice (plural) are mentioned once in the Dragons: Race to the Edge, Season 5 episode "A Matter of Perspective".

Description
Lice are tiny flightless insects of the Order Phthiraptera. Like all insects, they have six legs and three body sections - the head, thorax, and abdomen. Generally they are white, grey, tan, or other pale off-white colors. Lice species are almost all species-specific, meaning that, for example, lice from a Yak cannot survive on a human, only other yaks.

Lice can be divided into two categories - "chewing" lice and "sucking" lice. Chewing lice eat dead skin and other tidbits on the surface of the host animal's hide. Sucking lice eat bodily fluids and secretions on the skin surface.

Function
There is no actual use of lice in the Franchise or reality. They are, however, a pest species, and can adversely effect their host. Some species can transmit various diseases and a heavy population can cause skin damage and other effects.

Dragons: Race to the Edge, Season 5
In the episode, "A Matter of Perspective", Tuffnut tried to read Hiccup's mind and spouts of various things such as "Yak lice". Hiccup is not amused.

"Yes. Yak barf. Yak lice. Yak nose. Yak breath. Rocks. Water. Trees."

- Tuffnut