Learn Russian: Мы испугались волка, рычавшего на папину лошадь. - We got scared of the wolf growling at Dad's horse.

рычавшего is the past active participle of рычать (to growl). If we used a который clause, the sentence would be: Мы испугались волка, который рычал на папину лошадь. The participle must match the noun it describes (волка) in gender, number, and case, which is why it takes the masculine singular genitive ending -его.

Why is волка in the genitive case instead of the accusative?

Verbs expressing fear, like the reflexive испугаться (to get scared) and бояться (to be afraid), require their object to be in the genitive case. Even though the wolf feels like the direct object of our fear, Russian grammar demands the genitive here.

Why does the phrase на папину лошадь use the accusative case?

The preposition на takes the prepositional case when describing location (being on something), but it takes the accusative case for direction or the target of an action. Since the wolf is directing its growling at the horse, на takes the accusative.

What type of word is папину and how is it formed?