Those creatures are beasts
The creatures you refer to do not have a proper stat block. Rather, they're listed as "Chimeric creations", which can be selected from a table. The creatures are listed as based on an existing beast, as you've correctly noticed. For example, the cat you mentioned is described as follows :
A cat with antennae that grant it blindsight and tremorsense out to a range of 60 feet.
The rest of the text does not give additional information, only designating it as "the creature" or "one-of-a-kind pet" in the rest of the relevant text.
The closest match with official monsters I could find would be the Chimera, which is classified as a "monstrosity". However, apart from the name, nothing else explicitely links the Chimera to those Chimera-like creatures, so we cannot conclude on their creature type just with this information.
As such, since it is based off a regular creature, you keep that creature's original creature type. This means that your chimeric pet is technically a beast.
As such, assuming your character has already seen said creature, which means having them visit this specific location in this specific adventure module, RAW it is possible to transform into those Chimeric creations.
This is quite a unique situation : ask your DM
Those artificial, one-of-a-kind creatures are basically beasts that you could already transform into, with additional abilities added on top. This means that allowing a druid to turn into them is giving them more tools for solving problems.
As such, depending on the situation where this rule is invoked (especially if used in another module, after running this adventure), it could have unexpected side effects, ranging from not changing much, to completely breaking a scenario.
Either way, because this is such an exotic use of Wild Shape, the best course of action is to discuss it with your DM. Only they can decide whether to allow it or not.