DOOM Addon Glitch

When I spawned the Cyberdemon and shot him with a harpoon he changed into this:

What the hell is that thing?

It’s a Gargantua from Half Life 1

http://72.232.229.42/thumb/6/68/HalfLife_Gargantua.jpg/175px-HalfLife_Gargantua.jpg

-snip-

Ok, thanks I was wondering what it was. I just thought that was sorta weird.