No, it's true, there is such a thing as the golden warthog...though it is nearly impossible to get, and isn't even worth it
In one level, there is a sign with a golden warthog, and what you have to do is get 16 players (This is the nearly impossible part, because how are you going to get 16 people to agree to this without shooting each other)
Then, once you get the 16 people, everyone has to have a sniper rifle, and aim at this one specific spot on the sign
All 16 people have to shoot it at the same exact time, and when they do, a golden warthog appears....
There isn't really anything special about this warthog, other than that it is gold, and has no weapon at all in the back seat...
I remember reading something aobut that if the 16 people shoot it with a rocket launcher at the same time, then all weapons you pick up have unlimited ammo, but this isn't confirmed