Rian van Gend

Chosen answer: While no official explanation is ever given in the films, Alien canon suggests that the quadrupedal "Runner" Aliens (sometimes called "Dog-Aliens" or "Ox-Aliens" because of their animal hosts) are a weaker xenomorph variant that are physically unable to chest-burst and instead mature inside the host, eventually killing their hosts as they emerge later. This weaker xenomorph variant relies on four-legged speed, sneak attacks and the ability to spit acid at a distance, rather than on brute-force confrontations.

Charles Austin Miller