。
Two sheep were killed by a wolf.
Literal
Sheep [subject-が] 2-[counter-匹] wolf [by-に] were-killed.
匹 is the counter for small/medium animals (dogs, cats, sheep, fish, insects). The floating counter ２匹 is positioned between the noun phrase and the verb. 殺される is the passive of 殺す ('to kill'); the agent takes に.