What is a sentient being
In Classical Chinese Sutra, sentient being is 有情眾生, or simply 有情, a brief translation is one who has feelings, emotional beings. Trees, rocks, mountains... are not. But they can be dwelling places of Devas (Avatamsaka Sutra). I think in between there are marginal or transitory, like bacteria, slug, insect... like a rainbow though we say there are 7 colours, but in fact it's indivisible, between red and orange there is red-orange, re-red-orange, red-red-red-orange... so on and so forth, so the same as sentient beings and non-sentient beings, in respect to the multitude of lifeforms and substances.
How do you teach enlightenment to a cat ?
You should teach a human before teaching a cat, you should teach a human who has confidence and respect of Buddha before teaching those not... if a cat is ready for enlightenment, it's the Bodhisattva's job, not unenlightened human being you or me. Also a cat the first task before anything else, is to acquire human form - to be reborn as human.
Are the snow lions in a pure land enlightened ?
No snow lions in Pureland. Pureland is the emanation of Amitabha only those harmonious with his emanation are emanated by him, there are birds, trees, lotus, pavilions, clothes, foods, musics, dances, ponds... you may read the Pureland Three Sutras.
How do slugs get to be enlightened ? Do they have to be reborn as human first ?
There are no slugs in Pureland, all crawling things are not found in Pureland, if the birds are there it's due to Amitabha emanated them, and they are beautiful birds singing beautiful songs of the Dharma: the Four Truth.
It's not easy to transmit the Buddhist teaching if one didn't get a more profound understanding of what Buddha tried to convey, that the appearance of our world is purely our own conceiving. Such as this world - earth, with the grits, thorns, thunderstorms and lightnings, putrid smells, walking on two feet from distance to distance - spacetime... are all our own conceiving.