IThorny.cs

using System;
using System.Collections.Generic;

abstract class Animal
{
    public string Name { get; set; }
}

interface IThorny
{
    void Poke(Animal toBePoked);
}

class Hedgehog : Animal, IThorny
{
    public void Poke(Animal toBePoked)
    {
        Console.WriteLine($"The hedgehog {this.Name} has poked {toBePoked.Name} with those tiny thorns hedgehogs have.");
    }
}

class ThornyDragon : Animal, IThorny
{
    public void Poke(Animal toBePoked)
    {
        Console.WriteLine($"{this.Name} the thorny dragon slaps {toBePoked.Name} with a thorny tail.");
    }
}

class Dog : Animal { /* Dog actions */ }
class Rabbit : Animal { /* Rabbit actions */ }

class Program
{
    static void Main(string[] args)
    {
        var animalArmy = new List<Animal>
        {
            new Hedgehog { Name = "Shadow" },
            new ThornyDragon { Name = "Puff" },
            new Dog { Name = "Phillip" },
            new Rabbit { Name = "Fluffy" }
        };

        var thornyArmy = new List<IThorny>
        {
            new Hedgehog { Name = "Sonic" },
            new ThornyDragon { Name = "Pickles" }
        };

        animalArmy.ForEach(animal =>
        {
            // Can't do this!
            // Because this army is of Animal types you don't know which ones are thorny and therefore
            //   you don't know which ones can poke!
            //animal.Poke(thornyArmy[0]);
        });

        thornyArmy.ForEach(thorny =>
        {
            // Can do this!
            // Because this army is only made of thornies (IThorny) you know for sure they all
            //   can poke animals!
            thorny.Poke(animalArmy[0]);
        });
    }
}