The only giant slaughterfish was during a quest I did recently, but I defeated it and it won't respawn

I checked out it's ID and decided to spawn it back in-game (here is the info on the giant slaughterfish: http://oblivion.wikia.com/wiki/Giant_Slaughterfish and http://www.uesp.net/wiki/Oblivion:Giant_Slaughterfish )
However, when I spawned it (I tested it out just spawning it next to my house in anvil) all it was was a regular slaughterfish and NOT the giant one I remember facing during that quest.
So, is there a way to get the Giant Slaughterfish back in-game? I was hoping to add it to the levelled list in the CS so that you have a 75% chance of encountering it.
If no one else has done so, I might even just make this into a little mod that adds several giant slaughterfish in-game. But firstly, I need to know how to get it back.