This is a three-toed sloth. You will probably have to click to enlarge the photo to see him well. They are common in Costa Rica, but can be difficult to see because they usually stay high up in trees in the thick brush, hidden by lots of leaves.
Adding to the challenge in spotting them is that they spend about 18 hours a day sleeping, and even when they move, they move very slowly. On the ground, they typically travel at 1.5 meters (about 5 feet) per minute, although a female responding to the call of her baby has been clocked at three times that speed, or 4.5 meters or 14 feet per minute.
The reason that sloths are so, well, slothful, is that their diet consists of leaves that lack nutritional value and are difficult to digest, so they simply don't have much energy. More than half of their body weight of 4 kg (9 lbs.)consists of their stomach, which has multiple compartments to try to break down and digest the leaves that they eat. It can take their stomachs a month to complete the digestive process.
There is also a two-toed sloth, but it is less common and much more difficult to see. It is nocturnal, whereas the three-toed sloth is active, if you can use that word with sloths, both day and night.
The fur of sloths often appears to have a green coloration. That color is not their fur, but is algae growing in their fur. Their fur provides a mini-ecosystem for insects and algae, and the green coloration helps provide them camouflage in the trees.
Jaguars and eagles are their natural predators, and they are especially vulnerable when they are on the ground.
They spend long periods of time in the same tree. Studies have tracked that they change trees, going from branch to overhanging branch, about once every two days. They come to the ground about once per week for about a half hour. During that time they urinate, dig a small hole, and defecate. Why they put themselves at risk for such purposes is a mystery.
The mystery on why they come down for the above purpose is made more puzzling by the fact that female sloths even give birth while hanging in a tree. They are so lazy that they can miss their annual mating season because it may take longer than a year for a female and male to find each other because they spend the year hanging out (literally) in separate trees.
They are so lazy that some mother sloths whose babies lose their grip and fall to the ground may not even muster the energy to climb out of the tree to go retrieve their baby! I can't imagine another mammal whose laziness is stronger than the instinct to mate or to care for one's young.