The other yet unknown endings are of the third declension.

The third declension is very common. At first it may be hard to distinguish its nouns from nouns that start with -is (brolis, Laimis, rašiklis, etc), because they also have -is in the nominative case.

Let's see two examples: vagis (thief; masculine), rūšis (sort, kind; feminine)

Vagis is a masculine word, so it has an iui ending in the singular dative case. Feminine words (like rūšis) takes the ending iai.

Tas greitas vagis bėga namo! That quick thief is running home!
Yra tik viena kačių rūšis! There is only one kind of cats!

Du vagys važiuoja autobusu per tiltą. Two thieves are driving on a bus across a bridge.
Aš rašau apie penkias automobilių rūšis. I am writing about five kinds of cars.