Tresus capax is a species of saltwater clam, marine bivalve mollusk, common name the fat gaper, in the family Mactridae. It also shares the common name horse clam with Tresus nuttallii a species which is similar in morphology and lifestyle. Both species are somewhat similar to the geoduck (Panopea generosa, which is in the family Hiatellidae), though smaller, with shells up to eight inches long (20 cm), weight to 3–4 lb (1.4–1.8 kg).

The two species commonly known as horse clams inhabit the Pacific coast intertidal zones: the pacific gaper, T. nuttallii, more abundant south to California; and the fat gaper, T. capax, more abundant north to Alaska. Both have oval and chalky-white or yellow shells with patches of brown periostracum (leather-like skin) on the shell. These clams are also commonly called gapers because their shells are flared around the siphon and do not completely close, rather like geoduck clams. Like geoducks, they are unable to completely retract the siphon within the shell, though less flagrantly as the siphon on Tresus species is not as large.