There is an image of two Nanodragons in The Incomplete Book of Dragons that are not named or discussed. They are Nanodragons, because they are shown in that section of the Book which discusses Nanodragons.
These two are very similar, with the main feature being that they have a snail's shell as part of their bodies. However, one dragon has two eyes and the other has three eyes, each on stalk like a snail. Whether these are meant to be the same species or two related species, we may never know.
Unknown Cave Dragon
Under the Cave dragons section in The Incomplete Book of Dragons the dragon pictured at right appears. It looks a tad like a deep sea angler fish. Its appearance doesn't match any known Cave Dragons, nor is it named.
Unknown Sea Dragons
In How to Fight a Dragon's Fury, two separate pictures of similar Sea Dragons appear. It is unclear which species these are meant to be. They are not Seadragonus Giganticus Maximus, Woden's Nightmare, or Thor's Thunderer because they appear on the same pages with these species. These dragons do not match up with other known Sea Dragon species that have been described previously, such as the Leviathorgan. However, Savagers are mentioned multiple times in name only, so its possible these dragons are Savagers. Also possible: They are a new undescribed species, or the author drew them with known species in mind but drew them completely different.
Unknown Hunting Dragon
There is a Hunting Dragon mentioned in both How to Train Your Dragon and How to Be a Pirate, named Brightclaw. This dragon belongs to an unknown young Hooligan Viking. No information is given about its species or classification.
According to the official How to Train Your Dragon Books Website, some of the Viking Novices have Hunting Dragons that are not described. It is possible Brightclaw could be an Eaglefly or a Lackwit. However, Slitherfangs are also hunting dragons used by the Hairy Hooligan Tribe. Either of these three are possible, or Brightclaw could be an entirely different species altogether.
- Eaglefly and Lackwit in the Hiccup and Friends Guide on the Official How to Train Your Dragon Books Website.