The primary meaning of Ted Hughes's poem "The Jaguar," as I understand it, is that animals do not belong in zoos, and yet humans are continually drawn to and "mesmerized" by animals like the mighty jaguar.
The first several stanzas of Hughes's poem emphasize the "indolence," laziness, and general lassitude of most of the animals in the zoo. The parrots are strutting about, trying to attract the attention of passing visitors, hoping to be fed. Meanwhile, the lion, tiger, and boa constrictor are "fatigued" and lie so still that the cages appear empty to those who pass. These animals are not able to fulfil their natural roles in this zoo. As such, the fact that they are in a zoo is of no benefit either to them or to the humans who come to see them and are bored by their stillness. Why do we cage animals, when it is to the detriment of the animals themselves and causes them to behave in a way that is uninteresting to human beings, their wildness stamped out?
The jaguar, of course, is an exception in this instance—the passing children are desperate to "stare" at him, because the "freedom" in his stride is still visible and palpable, despite the fact that he is caged. The jaguar still "spins" around his enclosure: his nature is such that it cannot be trampled underneath the heel of imprisonment.
Still, it is clear that this is imprisonment for the jaguar. The animal is kept in "prison darkness," on a "short fierce fuse." He is still mobile not because he is in his natural habitat but because he is "enraged." Like a "visionary," he can see what lies beyond the bars; he continues to move as if he will one day escape. In some ways, though, this is even sadder than the indolence of the lions who have accepted their fate. The caged jaguar, a magnificent animal filled with fury, will never be returned to the "wildernesses" of which he dreams.