HorseComponentSetup.cs

using UnityEngine;

public class Animal<T> : MonoBehaviour
{
    public new T Injector;
}

public class Horse : Animal<ISteroidsInjector>
{
}

public interface IDrugInjector
{
    void Inject();
}

public interface ISteroidsInjector : IDrugInjector
{
    void InjectHarder();
}

public class TrienoloneInjector : ISteroidsInjector
{
    public void Inject()
    {
        Debug.LogError( "A pinch" );
    }

    public void InjectHarder()
    {
        Debug.LogError( "Ouch, that hurts" );
    }
}

public class HorseComponentSetup
{
    [RuntimeInitializeOnLoadMethod] public static void Setup()
    {
        SetupComponent<Horse, ISteroidsInjector>( new GameObject( "Horse Owner" ), new TrienoloneInjector() );
    }

    static void SetupComponent<T, TU>( GameObject owner, TU injector )
        where T : Animal<TU>
        where TU : IDrugInjector
    {
        var c = owner.AddComponent<T>();
        c.Injector = injector;
        c.Injector.Inject();

        ((ISteroidsInjector)c.Injector).InjectHarder();
    }
}