Corrected entry: In the other alien movies the chestburster's head is not yet formed the way the warriors' heads are, yet the chestburster from the dog (or, in the director's cut, the ox) has its head shaped like the warriors' heads, not the heads of the chestbursters seen in the other movies.
Shay
18th Mar 2005
Correction: Ripley mentions in a scene in the medical bay that this alien is different to the others (possibly because it gestated in a dog/ox rather than a human), so the fact that it's at a different stage of development when it's 'born' simply supports this further.
Shay