Pets like this:
Are still using vanilla era models and textures.
Even as recently as BfA we have whelplings using the exact same model:
As you can see, they’re both using the old outdated whelping model despite the fact that the new updated model was released in Legion which was before BfA:
Now you might say that this is because pets like the Twilight Whelpling may have been introduced in BfA but were in a Cata era zone. But I can refute this since pets like this exist and are from Island expeditions in BfA and still use the old model:
There is precedent for Blizz retroactively updating old pet models. I believe it was some time during BfA that they updated a lot of the old frog models and a few others I forget.
As an aside, I note there is a 3rd whelpling model that is also more updated than the original version: