Well, they are a bit complicated.
These nouns "decline", which is to say they change for to convey information.
In this case, the ending signifies that it is in the genitive case, so it signifies that the wolves belong to the Unsullied.
Nouns decline on case, of which there are 6 different cases, and number, of which there are 4 different numbers. This also is different declensions for each of the 4 genders, so different words will use different endings to signify the same thing (hontī showing the accusative singular vs abre showing the accusative singular)
If you visit Dothraki.org you can check out the different declension tables.