Carlastyanax is a monospecific genus of freshwater ray-finned fish belonging to the family Stevardiidae. The only species in this genus is Carlastyanax aurocaudatus, a characin, also known as the gold-tailed tetra, which is endemic to the upper part of the Rio Cauca drainage basin in Colombia.

The coloration of this species - a greenish-yellow back, a white belly, and fins that range from red to yellow - is notable for differing with the sex of the individual fish, a trait known as sexual dichromatism. Females have yellow-gold tail fins (which is where the species gets its name), and males have reddish or pinkish tail fins. Spawning occurs during various times of year in the chosen habitat of C. aurocaudatus, which is clearwater streams with organic or rocky substrate. Its range is relatively restricted, but is also home to the benthic insects and riparian plant material that make up its diet.

First named by German-American ichthyologist Carl H. Eigenmann in 1913, C. aurocaudatus has undergone rigorous taxonomic evaluation to determine its generic placement. Upon its nomination, it was considered to belong to the genus Astyanax, under the name Astyanax aurocaudatus.

In 1972, French ichthyologist Jacques Géry constructed a new genus for it, based on several features, including number and shape of teeth, the presence of an adipose fin, and number of gill rakers; specifically, one of the more important characteristics was said to be the presence of a hooked third dentary tooth. Carlastyanax was then re-synonymized with Astyanax in 2005 and 2006 by researchers, based upon morphological similarities more consistent with Astyanax than with a new genus, including the tooth.

However, more researchers in 2013 resurrected Carlastyanax from synonymy given further differentiating features, disregarding the hooked third dentary tooth. These include four teeth on the posterior premaxillary row (vs. five in all Astyanax) and eight branched dorsal-fin rays (vs. nine). A 2015 study of molecular phylogeny within the subfamily Stevardiinae (within Characidae) reaffirmed this decision. Today, Carlastyanax is considered a valid genus, and C. aurocaudatus its only species.

In 2017, the genus Carlastyanax was determined to be a part of a clade with genera Creagrutus and Piabina. However, in 2024, this taxon was placed in the subfamily Creagrutinae of the family Stevardiidae, within the suborder Characoidei of the order Characiformes.

Carlastyanax aurocaudatus is sometimes known as the gold-tailed tetra. In fact, its specific epithet means "gold tail"; "auro-" means "gold", and "caudatus" means "tail". The genus name "Carlastyanax" is to honor Carl H. Eigenmann and the original genus to which C. audocaudatus once belonged, Astyanax.

Carlastyanax aurocaudatus usually reaches a size of 6.0 cm (2.3 in) standard length (SL). Larger specimens can reach 7.4 cm (2.9 in) SL. The dorsal fin has 8 rays, the pelvic fin 5–6, and the anal fin 26–28. The head is large, with a blunt snout, and the body is generally robust. The base scale color is white ventrally, with a green-to-yellow predorsal region and a lateral stripe in silver or silver-blue. The humeral spot is usually blue, and is elongated vertically, as well as somewhat obscure. The pectoral and pelvic fins are pink or red with a dark margin. The other fins are different colors depending on whether the individual specimen is female or male.