If the aliens had been created, then the pure alien i guess would be the first perfected alien ever made. If the aliens evolved, then there is no pure alien of the kind you mean, because they have to had evolved from something that also impregnated hoasts, because such a stage of evolution doesn`t just come around, just like a type of shark evolved form another type of shark, it didn`t just appear all of a sudden, and as you can see, only sharks have continuously growing rows teeth, and so it has been for numerous stages of evolution. So what i`m saying is that the first xenomorph also came out of a hoast, it just came out being a little different from the ones before it, because of adaptation and evolution. So if you wanna find the pure xenomorph, you have to find the least evolved of the former generations that has most of the features a xenomorph has, except it was born in stead of impregnated, but still was the first generation to get the impregnating thing going. Just like the pure form of a human is the Homo Habilis or so. It`s still human, just not as evolved, and the ones before it were almost humans, but were still almost monkey too.
Like the Newborn, that was the pure form of a new species that could have evolved, good thing it didn`t get the chance to reproduce, just imagine how it would be to see those things walking on the walls in stead of the aliens...