In lowland South America the relationship between a shaman and a jaguar is not metaphor and is not possession. It is identity, and the languages say so. In several Tupian and Panoan languages the word for a powerful shaman is built from the word for jaguar, and the ethnographic literature records the expectation that a shaman becomes one, in this life or after death, and that a jaguar seen near a village may be a person.
This is why the jaguar carries an ambivalence no other American animal has. It is the source of a shaman's power and the thing a village most fears, and those are the same statement. A shaman who can become a jaguar can hunt for the community or hunt the community, and accusations of sorcery in Amazonian societies frequently take exactly that form.
The image is very old. Olmec sculpture from the second millennium BCE returns repeatedly to a figure combining an infant with a snarling feline mouth, the so-called were-jaguar, and while what the Olmec meant by it is not recoverable, the continuity of the theme through Maya and Aztec material is not in doubt. Maya rulers used jaguar names, sat on jaguar-skin cushions and were buried with jaguar pelts. The Aztec ocelotl warriors wore the skin whole, head over head.
What holds the two continents together is not a shared myth but a shared reading of the animal: solitary, nocturnal, capable of killing anything including a person, and largely unseen. A god who behaves that way needs no explanation.