pytest-mockでなくても簡単な置き換えはpytestのmonkeypatchで可能

animal.py

def get_animal():
    return "cat"

def get_my_pet():
    return get_animal()

tests/test_animal.py

from animal import get_my_pet

def test_get_animal():
    assert "cat" == get_my_pet() # 通常はcatが返る

def test_get_animal_fake(monkeypatch):
    # 置き換える関数を作成
    def get_animal_fake():
        return "dog"

    # 関数を置き換える
    monkeypatch.setattr("animal.get_animal", get_animal_fake)
    assert "dog" == get_my_pet() # 置き換えた関数の返値