Animals

public class Animals : IEnumerable<Animal>
{
    public void Add<T>(T animal) where T : Animal
    {
        var type = typeof(T);
        if (!_animals.ContainsKey(type)) _animals[type] = new List<Animal>();
        _animals[type].Add(animal);
    }

    public IEnumerable<T> GetAll<T>() where T : Animal
    {
        var type = typeof(T);
        return _animals.ContainsKey(type) ? (IEnumerable<T>) _animals[type] : Enumerable.Empty<T>();
    }

    public IEnumerable<Animal> GetAll()
    {
        var animals = new List<Animal>();
        foreach (var kvp in _animals)
            animals.AddRange(kvp.Value);
        return animals;
    }

    Dictionary<Type, List<Animal>> _animals = new Dictionary<Type,  List<Animal>>();

    public IEnumerator<Animal> GetEnumerator() => GetAll().GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}