Ok so that error means its missing a namespace... at the top of the script it has a bunch of "using" this and that. You need to put in the right one into the script so that the code will compile.
SO this is the rainbow savage ridgeback...
using System;
using Server.Mobiles;
namespace Server.Mobiles
And this is the default Beetle where i got that code so its got the namespaces at the top u need...
using System;
using System.Collections.Generic;
using Server.ContextMenus;
using Server.Items;
namespace Server.Mobiles
As you can see the beetle is using more namespaces so you need to add them to the rainbow mount to make it work... so at the top of the rainbow mount replace it like this...
using System;
using Server.Mobiles;
using System.Collections.Generic;
using Server.ContextMenus;
using Server.Items;
namespace Server.Mobiles
It compiled no problem for me after that.
So what you want to do is add all of the code from beetle.cs. I did this and it still wasnt working but this is what you must do for it to behave correctly. its not hard takes little time with notepad ++ and the find feature and compare feature. Most imporatant is to add this code in its entirety... Copied from beetle.cs...
#region Pack Animal Methods
public override bool OnBeforeDeath()
{
if (!base.OnBeforeDeath())
return false;
PackAnimal.CombineBackpacks(this);
return true;
}
public override DeathMoveResult GetInventoryMoveResultFor(Item item)
{
return DeathMoveResult.MoveToCorpse;
}
public override bool IsSnoop(Mobile from)
{
if (PackAnimal.CheckAccess(this, from))
return false;
return base.IsSnoop(from);
}
public override bool OnDragDrop(Mobile from, Item item)
{
if (CheckFeed(from, item))
return true;
if (PackAnimal.CheckAccess(this, from))
{
AddToBackpack(item);
return true;
}
return base.OnDragDrop(from, item);
}
public override bool CheckNonlocalDrop(Mobile from, Item item, Item target)
{
return PackAnimal.CheckAccess(this, from);
}
public override bool CheckNonlocalLift(Mobile from, Item item)
{
return PackAnimal.CheckAccess(this, from);
}
public override void GetContextMenuEntries(Mobile from, List<ContextMenuEntry> list)
{
base.GetContextMenuEntries(from, list);
PackAnimal.GetContextMenuEntries(this, from, list);
}
#endregion
After I did that I enabled the backpack in the context menu... Dont forget to copy all of the code over into the right spots and then also the namespaces at the top for that specific error message you got...
I'm trying to get the rainbow steed to have a backpack like the pack mule does. I did try a few things and welp it didn't go to well for me 
