The fossa, the largest carnivorous mammal, exclusively found on the island of Madagascar, are considered to have descended from Mongoose that arrived on the island of Madagascar from mainland Africa, around 24 million years ago. However, they look like a cross between a cat, a dog and a mongoose. With slender bodies, muscular limbs and short, reddish-brown coats, they have small, cat-like heads, short, dog-like muzzles, large, rounded ears like a weasel and a long tail like a monkey.

Usually the fossa adults weigh between 5.5 and 8.5 kg, with the males larger than the females. They grow to a length of about 5 feet (1.5 m), including a tail about 26 inches (66 cm) long and have short legs and sharp, retractile claws, which they can extend but not retract fully. The long and slender tail, which makes up about half of the animal's length, provides balance as it moves through the trees and comes in handy while hunting and maneuvering amongst the tree branches. The retractile claws and flexible ankles allow them to climb up and down trees head-first. However, like the bears, they walk flatfooted on the soles of their feet.

The genus name of the fossa is Cryptoprocta, as their anus (procta) is hidden (crypto) by an anal pouch. The male fossa has an unusually long penis and penis bone, with an average thickness of 0.79 inches (20 mm), reaching to between his forelegs when erect. However, during adolescence, the female fossa undergoes a peculiar developmental stage, which is known as transient masculinization. She develops an enlarged, spiky clitoris that resembles the male's penis and secretes an orange substance that colors their underparts, much like the secretions of males. The enlarged clitoris, decreases in size as the animal grows.

The fossas prefer to stay in forested areas with ample food source and plenty of space. Unfortunately, the growing human settlements in Madagascar have severely affected their population, as they are often hunted by farmers for the safety of their livestock. With the exception of the breeding season, the elusive fossa is a solitary animal, spends its time both in the trees and on the ground and patrol to defend territories as large as four square kilometres. Both males and females mark their territories by the scented secretions from glands on their chests and under the base of their tails. They hunt during the day and night alike and can take prey from both the ground and in trees. However, their activity peaks may occur early in the morning, late in the afternoon and late in the night. They prey on small- to medium-sized animals from fish to birds, rodents, lizards, frogs and wild pigs. However, lemurs are their main food source and they are the only predators in Madagascar, who are capable to kill the largest lemur species.

Like many other carnivorous mammals, Fossas only come together to mate during the breeding season between September and December. They have a polyandrous mating system, in which a female takes two or more males at the same time. They mainly make sounds to express themselves during the breeding season, when the females mew to attract males and the males sigh when they find a receptive female and yowl when competing for her affections.

Though occasionally they mate on the ground, usually the receptive female occupies a site high in a tree, often near a water source, strong and wide enough to support the mating pair, while the males gather below and wait for their turn. The female may choose to mate with several of her suitors over the next week. The bouts of copulation may last for several hours, due to the backwards-pointing spines along most of the length of the male's erect penis, which results in a copulatory tie that is difficult to break. Copulation with a single male may be repeated several times, with a total mating time extended up to fourteen hours and the female may occupy the tree for up to a week, mating with multiple males over that time. After her departure, another female may take her place, mating with some of the same males. Strangely enough, the same trees are frequently used year after year and this type of mating system, in which a female monopolizes a site and chooses her mates, seems to be unique.

Just before the completion of gestation period that lasts for around three months, the carrying female fuss makes a den in a place like an old termite mound, underground den, a rock cave or the hollow of a tree. Two to six toothless and closed eyed pups are born in a litter and the mother raises them without any help from others. The pups do not open their eyes until they are between two and three weeks old, when their fur begins to darken. They develop slowly and do not leave the den for about four to five months. Fossa youngsters continue to grow until about two years of age and can live up to 17 tears.