This is the 12712th most frequent Russian word.
косулю
"Косулю" is the accusative form of "косуля," meaning "roe deer" in English.
Here, 'косулю' is in the accusative case, as it is the direct object of the verb 'увидел' (saw).
Я увидел в лесу косулю.
I saw a roe deer in the forest.
In this sentence, 'косули' is in the dative case, indicating the recipient of the action.
Для косули мы оставили немного хлеба.
For the roe deer, we left some bread.
Here, 'косули' is in the genitive case, showing possession or relation.