Capybara
The largest rodent alive, and a semi aquatic one: it swims, and it grazes. South American herds run to a dozen or more, so one kept on its own does badly.
Overview
The largest rodent alive is also a swimmer. A capybara runs 35 to 66 kilos and 106 to 134 centimetres long, stands about 60 centimetres at the shoulder, and its body is only slightly denser than water, which is why it floats so easily and can stay completely under for up to five minutes.
It is built around a herd and around a pond, in that order. Wild groups sit at about ten adults and stretch from three to thirty when the dry season pushes them together at whatever water is left. They are crepuscular, busiest at dawn and dusk, and they spend the hot middle of the day in the water because they overheat easily. One kept alone, on dry ground, is an animal with nothing it is designed to do.
Temperament
Placid with people and dependent on its own kind. The calm that made the capybara famous online is a herd animal's calm, and it does not survive isolation intact.
They are vocal in a low, constant way, with whistles, clicks and purrs passing back and forth between animals that are grazing near each other. A single capybara has nobody to say any of it to. Plan for a pair at minimum and understand that the second animal is not an upgrade, it is the requirement.
Care and grooming
Grass, and a lot of it. Capybaras are grazers, taking grasses and aquatic plants with some bark and fruit, and the bulk of the diet has to be fibre rather than produce.
The part people are not ready for is coprophagy. A capybara spends part of every morning re-eating the previous day's droppings, which is how it gets the second pass at fibre its gut needs. It is normal, it is necessary, and interrupting it does the animal harm.
Living and exercise
Water deep enough to swim in, not a paddling pool. Swimming is not enrichment for this animal, it is thermoregulation and it is where it goes when anything frightens it.
Then grazing space around it, shade for the middle of the day, and fencing that accounts for something that weighs as much as a person and pushes. Ownership rules differ by state in the US and it is worth checking yours before anything else, since several states require a permit and some do not allow them at all.
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