A fossa is about the size of a medium dog - around 80 centimetres long, plus a tail just as long. Its fur is a reddish-brown colour and very smooth. It has a long body, short legs and a long tail it uses for balance.
Fossas are amazing climbers. They have ankles that twist all the way around, like an owl's neck, so they can run down a tree trunk head-first. Most cats and dogs cannot do that. It means a fossa can chase a lemur up into the canopy and through the branches without ever slowing down.
Because Madagascar has been alone for so long, no other big predators ever arrived to compete with the fossa. No lions, no leopards, no wolves. The fossa filled the 'top predator' job all on its own and got very good at it.
Fossas are shy and rarely seen by people. They mostly hunt at night and rest during the day. Even in Madagascar, you can spend weeks in the forest and not catch a glimpse of one. Scientists who study them often track them using tiny cameras tied to trees.