The Cuniculus paca, formerly known as Agouti paca is in the Order Rodentia under the Family Cuniculidae, which contains only one other species, the mountain paca (A. taczanowskii) (Wainwright 2007). The paca was formerly in the same family as the agouti in the Family Dasyproctidae.

Being the largest rodent in Costa Rica (Henderson 2002), the paca can be 70cm in length and weigh up to 9kg (28in., 20lbs.). The males are larger than females. The hind tracks are about 5cm wide and the skull length is about 19cm (Wainwright 2007). C. pacas are similar to the agouti and tapir, having a pig-like body shape with the upper reddish brown parts marked with horizontal rows of cream-colored spots along the sides. They are strictly nocturnal terrestrial mammals that live in burrows and hollow logs during the day (Smith 1983). As a defense mechanism against predators and to intimidate same-species rivals, the pacas can produce loud sounds of amplified grunts, growls, barks, and tooth-grinding noises. This is possible due to the unusually swollen zygomatic arch (the cheek bone) that acts as a resonating chamber, which is unique to mammals. Young pacas make a meowing sound until about a month old (Wainwright 2007).