Agree, the species is Hyphessobrycon eques, and it is seen under common names like Serpae, Red Minor. Its history is certainly varied with respect to taxonomy.
This species is closely related to some 30 Hyphessobrycon species [Weitzman & Palmer (1997) referred to this species group as the rosy tetra clade] that includes H. bentosi, H. rosaceus, H. megalopterus [Black Phantom], H. sweglesi and H. erythrostigma ["bleeding hearts"] to name but a few. It shares the common traits of a black dorsal fin and a dark (in this species black) humeral or shoulder blotch immediately posterior of the gillcover, and (usually) a red body colouration. The shade of red varies on fish from different geographical regions. The dorsal is edged in white. This species is readily discerned from all the other rosy tetra species by the dark colour pattern on the posterior part of the anal fin that expands from the black distal (furtherest from the body) border of this fin. Weitzman & Palmer (1997) note that this dark pattern is somewhat variable on wild-caught specimens from different locations.
Most of the fish of this species now available in the hobby are commercially raised and differ from wild-caught fish with respect to the dark shoulder or humeral patch. Commercially bred fish are descended from hybrids (perhaps unintentionally) of fish from different geographical areas, and the patch is shorter or all but absent on most; on wild-caught fish, this patch is black, elongate and slightly triangular.
This fish has gone through several name changes, being placed in four different genera before being assigned to Hyphessobrycon by Weitzman & Palmer (1997). Within this genus, the names H. callistus and H. serpae are now recognized as synonyms of H. eques and not valid distinct species; fish still appear in stores under these synonyms.
Authors that have recently studied the systematics of the genus Hyphessobrycon (and Hemigrammus, both of which hold wellover a hundred species) have unanimously pointed out that the group is not well defined and it is polyphyletic. [A monophyletic genus is one wherein the species share a common ancestor, thus linking them together physiologically; a polyphyletic genus has species descended from different ancestors so the species are less closely relateed.] Mirande (2009) for example has proposed several revisions to the family Characidae based upon phylogenetic diagnosis. Some genera have been moved to a new subfamily, while others are now (temporarily) assigned to a specific clade within the family pending further study. The recognition of groups of species [clades] within Hyphessobrycon is based primarily on similarities of color patterns; an hypothesis of its intra-relationships is currently unavailable. Hyphessobrycon has until recently been differentiated from Hemigrammus solely on the basis of the fish in Hemigrammus possessing a scaled caudal fin; this however is now known to be unreliable, since it occurs in intermediate conditions (de Lucina, 2003).