I am working on a filter where the users should be able to filter a list of products based on the animals the products can be used for. If the user selects 'dog' the products which are usable for dogs should be shown.
There is also a category of products which are usable for all animals. These should be shown regardless of which filter the user choose. So far so good.
The main issue here is that the animal classification of products is optional. There are products which have no animal set. These products could although be for specific animals or for all animals, we do not know.
The wording of the filter items does now get tricky. Would selecting "all animals" be equal to not filtering the list and showing all products? Or would selecting "all animals" equal showing the products which actually are for all animals?
And one more thing, the products are animal medicine so selecting eg. an elephant drug for a ginnypig should not be a mistake users can do. The all animals filter must therefore be very clear and intuitive.
Edit: it's a drop down on top of a list of products. The users can click on a product which takes them to a page with extensive information about the product.