This is the 14714th most frequent Russian word.
пес
Dog.
The word 'пес' is used as a noun here to refer to a male dog.
Эта пес лает громко ночью.
This dog barks loudly at night.
The word 'пса' is the genitive singular form of 'пес', indicating possessive or descriptive function.
Мы решили завести пса, чтобы он охранял двор.
We decided to get a dog to guard the yard.
The word 'псу' is the dative singular form of 'пес', used to indicate an indirect object or recipient.