List<Cat> cats = animals.Select(x => new Cat() { Name = x.Name, Age = x.Age, Color = null }).ToList();

class Program
	{
		static void Main(string[] args)
		{
			var _animals = new List<Animal>()
			{
				new Animal() { Name = "One", Age = 1 },
				new Animal() { Name = "Two", Age = 2 },
				new Animal() { Name = "Three", Age = 3}
			};

			var _cats = _animals.Select(x => new Cat(x).ToString()).ToArray();
			Console.WriteLine(String.Join("\n", _cats));
			Console.ReadLine();
		}
	}

	public class Animal
	{
		public string Name { get; set; }
		public int Age { get; set; }
	}

	public class Cat : Animal
	{
		public Cat(Animal a)
		{
			this.Name = a.Name;
			this.Age = a.Age;
			this.Color = "Полосатый";
		}

		public string Color { get; set; }

		public override string ToString()
		{
			return String.Format(
				"Name - {0}, Age - {1}, Color - {2}", this.Name, this.Age, this.Color);
		}
	}

public class Animal
	{
		public string Name { get; set; }
		public int Age { get; set; }

		public static explicit operator Cat(Animal a)
		{
			return new Cat() { Name = a.Name, Age = a.Age, Color = "В полоску" };
		}
	}

	public class Cat : Animal
	{
		public string Color { get; set; }

		public override string ToString()
		{
			return String.Format(
				"Name - {0}, Age - {1}, Color - {2}", this.Name, this.Age, this.Color);
		}
	}