What are examples of the "muse" in Keats poetry?

John Keats both addresses human muses as well as treats nature and inanimate objects as sources of inspiration. Some of his finest poems combine the two, as he addresses natural and cosmic phenomena but merges themes of eternal love into his admiration for them. The great love of Keats’ life was Frances (Fanny) Brawne, whom he met in 1818 and to whom he was engaged at the time of his death in Italy.
A number of poems, collectively referred to as the “Fanny lyrics,” are dedicated explicitly to her or implicitly evoke her. One is titled simply, “To Fanny.” The group includes "I Cry Your Mercy, Pity, Love—Ay, Love!" In this anguished poem, Keats speaks first to Love itself and then to his beloved, asking for her love and saying life is not worth living without it.

Yourself—your soul—in pity give me all,
Withhold no atom’s atom or I die
Or living on perhaps, your wretched thrall,
Forget, in the mist of idle misery,
Life’s purposes….

One of Keats’ most outstanding works, which brings together these two strands, is “Bright Star.” In it, he combines his reverence for the star always shining in the heavens with the constancy of true love, which he wishes always to maintain. The poet establishes this in the first line, “would I were steadfast as thou art.” From the heavens, the star would look down on earth’s glories, such as the waters on the shores, and the snow on the mountains. The poet, in contrast, would steadfastly lie with his beloved, “Pillow'd upon my fair love's ripening breast….” That would be all the constant perfection he ever needed:

Still, still to hear her tender-taken breath,
And so live ever—or else swoon to death.
https://www.poetryfoundation.org/poems/44468/bright-star-would-i-were-stedfast-as-thou-art

https://www.poetryfoundation.org/poems/50326/i-cry-your-mercy-pity-love-aye-love