var client = new MongoClient("mongodb://root:root@localhost:27017");
var collection = client.GetDatabase("test").GetCollection<Animal>("animals"); 
var dogs = collection.OfType<Dog>().Find(Builders<Dog>.Filter.Empty).ToList();  

[BsonKnownTypes(typeof(Dog))]
public abstract class Animal 
{ 
    public ObjectIdId { get; set; } 
}
public class Dog : Animal
{}