Pacas display a spotted pattern on a brownish or blackish ground color, but agoutis are usually uniformly colored above and pale below. Both groups have a piglike body and a rabbitlike head. The external form is modified for running in that the limbs, especially the hind legs, are lengthened and the lateral toes are reduced in size. The thumb is vestigial. The claws are thick and hooflike.
Females have four pairs of ventral mammae.
The incisors are fairly thin, and the cheek teeth are high-crowned and semirooted. Part of the zygomatic arch of the skull in Agouti is greatly enlarged and contains a large sinus, a condition not found in any other mammal. The surface of this enlargement becomes extremely rugose in the adult. This character is the principal basis on which some authorities place Agouti in its own family. The Johns Hopkins University Press.