system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Are normal animals (eg cow, turtle, birds) considered xenos that be exterminated? If not, where is the line drawn?
gpt
of course not. These are all animals that have been/ can be domesticated and serve as a food source for humans. 

The basic Categorization, as far as the Imperium is concerned, of a human is a creature with sentience, 2 arms, 2 legs, 20 digits, and a head. Therefore, we have designated ab-humans such as the Ogryn, Squats, Navigators, and Ratlings that are allowed to live within the Imperium and can fight alongside the base humans. 

As far as where the line is drawn. that really depends on the situation. For instance, Eldar are most certainly non-human, however they do somewhat resemble our species and thus the Imperium has deined it useful to join forces on rare occasians. However, the Eldar are still considered Xenos filth to be wiped out eventually.

One, non-bipedal xenos that comes to mind as far as the classification of exterminatus are the horrid, Megarachnids that the great legions stumbled upon during our Great crusade. Had these Xenos turned out to be just mindless creatures, even if they attacked the legions, they possibly would have been left alone, however their intelligence in their attacks and their social behaviour, deemed them a species that was to be exterminated.

If a race is intelligent, and does not look like us.  then it will be annihilated.