Cucumber

Cucumbers are mentioned very briefly in How to Train Your Dragon of the Book series.

Description
Cucumbers are vegetables typically green or yellow in color and are related to gourds, pumpkins and Squash. Cucumis sativus is the species cultivated by humans for food. They often have an oblong tubular shape, but other shapes exist, depending on the cultivar. Cucumbers are divided into three categories: pickling, seedless, and slicing; this is dependent on what the particular cultivar is best used for.

Cucumbers originated in India, but spread through Asia and Southern Europe through the Greeks and Romans. Cucumbers were known to be grown in France in the 9th century. This makes it plausible that Vikings may have come across cucumbers in their travels.

Function
The only use of a cucumber is for eating in various methods. They are also widely known as 'pickles' when preserved through the process of pickling (though anything that is pickled is known as a pickle as well).

In the Book series, cucumbers are suggested to be fed to Toothless, indicating dragons may be able to consume them safely.

How to Train Your Dragon
Cucumbers are mentioned by Snotlout in a derogatory fashion when he suggests feeding them to Toothless due to his lack of teeth.

"Has the Toothless Wonder gotten up the nerve to attack a vegetable? Carrots are a bit crunchy but perhaps he could manage the odd squished cucumber. ... You could give it to him though a straw perhaps. ..."

- Book 1