Leech

Leeches are animals mentioned in the How to Train Your Dragon Franchise.

Description
Leeches are invertebrates belonging to the phylum Annelida. They are segmented worms with two suckers, one at each end. Unlike other worms, their external segmentation does not correspond with the internal segmentation.

Most leeches live in freshwater environments, while others may live in terrestrial or even marine environments.

The most known species of leech is the Medicinal Leech (Hirudo medicinali), which is used in treatments for various diseases.

Function
There is no mentioned function for leeches in the How to Train Your Dragon Universe.

In reality, the leeches are used for medicinal purposes, especially reattachment of body parts and reconstructive and plastic surgeries. Their saliva is also used as a coagulant.

The Ice Castle
When Stoick finds out that Stormfly was taken by Arngrim Dammen, he says that the man is a lowlife leech.

Dragons: Race to the Edge, Season 1
Ruffnut mentions in When Darkness Falls that they once found a leech attached to Tuffnut. "Ruffnut: I don't know, I haven't seen him this freaked out since he found a leech on his- Astrid: Okay, I'm going to stop you there."

Dragons: Race to the Edge, Season 2
Tuffnut calls the parasites called Grimora leeches in the episode A Grim Retreat. "See ya later, ya leeches!"

Trivia

 * Grimoras appear to be based off the leeches, in terms of appearance and feeding behaviour.