Non-sentient creatures are the creatures who don't exactly have the same perception that we as humans do. They will be the background animals for your world, the filler creatures. There are a few different ways of populating your world with animals.

The first method is to use animals you can find on Earth. Most fantasy stories are set in worlds similar to our Earth in medieval times, so this method helps. Just imagine what animals will work for the geography you have, and populate accordingly.

Another method is to use fantasy creatures believed to exist during medieval times. This includes dragons, unicorns, manticores, etc. If you aren't sure what sort of creatures you can use from this category, look in an encyclopedia, under either "medieval" or "fantasy creatures" for examples.

A third method is to create your own. This especially helps if your world has exotic animal life, or has mutations or alien species as part of its existence. To do this, one of the best ways is to take parts of different animals and mix them together. If you have any inclination toward drawing, you might try to doodle some of your creatures. It helped me, when creating creatures to populate Tyther, even back when I couldn't draw nearly half as good. Simply sketch the basic ideas of what each creature looks like. It doesn't have to be perfect. If you can't draw, you might write out a brief description of each. Just make sure that you have good details for each creature, and that they could plausibly exist. Do not put a creature in the tundra that could not survive cold temperatures, or a creature in the desert that could not stand extreme heat. Don't put an animal in water that will drown (unless it's like a dolphin or a whale), and don't put a fish on land. Use common sense. Once you have created your creatures, name them. You could give them species names that describe what they are, or sound like the noise they make. The names could be done in the style of one or more of your world's languages. You decide.