We don't know much about the speaker, but we're willing to bet his favorite animal would be the sloth.
A big fan of lounging around and doing nothing (much like the sloth, who sleeps about 15-20 hours per day), the speaker spends most of the poem wondering why he shouldn't be allowed to be lazy. After all, it's more enjoyable for him than things like love, ambition, or even writing poetry, which he calls his "demon."
This "demon" talk might be a clue about the speaker's talents. He's tempted enough by poetry writing, and his own ambition, to consider (if only briefly) giving up the pleasure of doing nothing. Perhaps he feels that he is wasting his talents by ignoring his skill with poetry, or perhaps he's just tempted by the chance to win a little praise.
Either way, the sloth wins, as the speaker retires into the grass for a nice, long nap… at least, for awhile. After all, he did write this poem.
And whether not the speaker is supposed to be Keats, he is a poet who struggles with whether to write or just enjoy himself. We guess the speaker could stand in for any poet who wants a good excuse to sleep in every once in awhile.