Familiarize yourself with the unedited linearts and then look for the pose. It can be real tricky to figure out what the original lineart is for some edited pets but they typically (but not always) share the same pose.
Like these pigs are edited PPS cats:


Notice especially how the legs of the pig are all in the same position of the cat's legs.
And when you then have an idea for what species it is, look it up in the archive. In this case, filter by cat and you'll find the pigs show up too, that means they're actually cats.
A few other examples:


This hedgehog lineart is a syrian hamster. Again notice the legs and the general pose.


Adult cows are ponies. Again the legs.
And then a real tricky one, the dimetrodon!


Believe it or not but that's a rat! Essentially the only thing they have in common is the short stature and the overall body shape.
And like I said, the archive has the answers. If you filter by the species you think it is and it doesn't show up, then that's not the real species. If it does show up, then that's the real species.