"He sees a small sheep."
Translation:Vidí malou ovci.
This is the accusative case. There is nothing shocking here, the accusative case is the most important oblique case as it is used for most direct objects. You can't do too many useful sentences without direct objects.
See the Tips and notes about the adjective declension. You can also use the table at https://en.wiktionary.org/wiki/mal%C3%BD#Declension
You use the -ou ending when it is the appropriate choice for the gender of the word and its place in the sentence. If you have not done so, please read and check the links in VladaFu's comment above. You'll want the T&N for the Animals Skill. (Click the Skill, then click the light bulb icon; available on the web only currently.)