When considering purchasing a weighted animal, there are some things you should keep in mind.
Firstly, why are you buying the animal? These animals are designed to provide proprioceptive input which, to put it simply, is a means to provide sensations from joints, muscles and connective tissues that underlie body awareness creating stability, and have calming benefits to improve attention, focus and concentration. A properly made weighted animal is also a great tool for those with Tactile Defensiveness.
Now having got that out of the way, points to look for:
Size - the weighted animal is in many ways is like a lap blanket but much cooler, so it is best if the animal is around 70 cm long and can lay across the lap, animals such such as a platypus, lizard or alligator are ideal.
Weight - around 2 kg is an ideal weight, less than that weight for smaller children and heavier for older children and adults. The animal is best used resting on the lap, over the shoulder or on the chest. The weight should be spread evenly over the length of the animal (very important).
Material - the material is most important, a soft plush material is great for tactile awareness and has a calming effect. An animal with a fur covering is suitable for a child that shuts down and would find the fur stimulating. Different materials on the body provide a range of tactile inputs.
Still with me? OK, the look and shape of the animal is important, it should be an animal that is not too babyish, something like, as I said before, a platypus or lizard is in the "cool" zone. It shouldn't be too large as the time to use it maybe when the person is sitting at a table or desk.
I could go on but by now you should have an idea about what is important.