Taxonomy & naming
Mormyrus caballus was described by the Belgian-British ichthyologist George Albert Boulenger in 1898. It belongs to the family Mormyridae, the elephant-snout fishes, within the order Osteoglossiformes — the bonytongues, one of the oldest living lineages of teleost fish, named for a toothed bone on the floor of the mouth that bites against a matching pad on the roof. GBIF and the Catalog of Fishes both treat Mormyrus caballus as the current valid name.
The genus Mormyrus is one of the largest in the family, with more than twenty described species of elephant-snout fishes spread across African rivers and lakes; the family as a whole holds upward of 180 species, all weakly electric and all endemic to Africa, ranging from the Nile and the Congo to the Zambezi and the great lakes Victoria and Tanganyika. The genus name comes from the ancient Greek mormyros, originally the name of an unrelated marine sea bream and later carried over by naturalists to this freshwater group. The species name caballus is Latin for "horse," most likely a reference to the long, downturned, horse-like head profile shared by many Mormyrus species.
Morphology
Mormyrus caballus has the elongated, laterally compressed body typical of its genus: a long-based anal fin, a small dorsal fin set well back toward the tail, and a deeply forked caudal fin. FishBase lists a maximum size of 19.7 in total length, which is large for the family and considerably bigger than the small elephantnose species (Gnathonemus, Campylomormyrus and similar) more familiar in the aquarium trade.
As in all mormyrids, an electric organ near the base of the tail produces a weak, species-specific pulse-type electric-organ discharge. The fish reads the distortions this field picks up through electroreceptors covering the skin, a sense used to find prey and obstacles in dark water and, it is thought, to recognise other mormyrids. Mormyrids as a family are noted for some of the highest brain-to-body mass ratios of any fish, with an enlarged cerebellum devoted to processing this electrosensory input; whether this has been measured specifically in M. caballus isn't documented in the available sources, but it is a defining trait of the family it belongs to.
Habitat
The species is known from the lower and middle Congo River, from the river's mouth upstream to Wagenia (Boyoma) Falls in the Democratic Republic of the Congo, with an unconfirmed record from the Chiloango basin in Angola. FishBase and the U.S. Fish and Wildlife Service both place it firmly within the Congo drainage.
It lives on soft substrate over the floor of the river and its floodplain, in dark, dimly lit water typical of the lower Congo system. FishBase records the species from tropical freshwater around 72–75 °F; pH data specific to this population is not documented in the sources reviewed here, though mormyrids in general are associated with soft, slightly acidic to neutral, tannin-stained water — the kind of low-visibility habitat the electric sense evolved to read.
Feeding
Species-specific feeding data for Mormyrus caballus is sparse. Mormyrids as a family are nocturnal predators of small invertebrates, using their electric sense to locate prey buried in soft sediment after dark, and there is no reason to expect this species differs from that general pattern.
General guidance for elephant-snout fishes in the aquarium describes a diet of live and frozen foods — bloodworm and brine shrimp in particular — with prepared flake food rarely accepted. That advice comes from care literature covering the family rather than this species by name, so it should be read as a starting point rather than a confirmed diet for M. caballus.
Mating
Courtship in mormyrids is understood to rely on the electric-organ discharge: subtle differences in the pulse waveform and pattern are thought to let fish recognise species and sex and to play a role in courtship and dominance interactions. Beyond that broad picture, the mating behaviour of Mormyrus caballus specifically has not been documented in the sources available here.
In the wild, mormyrids are generally seasonal spawners tied to flood cycles, when rising water opens up floodplain habitat and food. Aquarium literature on the family notes that confinement in a tank appears to disrupt the electric signalling fish rely on to identify potential mates, which is one proposed reason captive spawning is so rarely seen — though the underlying mechanism is not fully worked out.
Breeding
Breeding of Mormyrus caballus in captivity is, as far as the available sources indicate, essentially unrecorded, and mormyrid breeding in general is poorly understood and only rarely achieved in home aquaria. The size the species reaches — up to 19.7 in — makes it an especially unlikely candidate for a home-aquarium spawning project even before considering the family's other breeding difficulties.
Some mormyrids are thought to guard eggs laid among plants or root tangles in the wild, but this has not been confirmed for M. caballus, and no aquarium spawning account for the species is documented here. Anyone encountering an alleged captive breeding of this fish should treat it as a notable, poorly documented event rather than a repeatable method.
In the aquarium
This is not a beginner's fish, and its adult size sets it apart from the small elephant-snout fishes the hobby usually keeps: at up to 19.7 in total length, Mormyrus caballus needs a considerably larger, more mature aquarium than the roughly 40-gallon minimum often quoted for smaller mormyrids, with soft substrate for probing, dim lighting, and secure hiding places for a fish that is most active after dark.
Mormyrids are consistently described as sensitive to water quality — sensitive enough that some species have been used as bioindicators in municipal water testing in Germany and the United States — and this species should be assumed to share that sensitivity until shown otherwise. Stable, pristine, well-filtered water and a quiet environment free of excess noise and vibration are recommended for the family generally.
Elephant-snout fishes are usually described as peaceful toward other fish but intolerant of their own kind in small numbers; care guidance for the family recommends keeping a group of five or more rather than a pair, to spread aggression rather than concentrate it on a single tankmate. Given this species' size, that means committing to a large, specialised setup rather than a single tank-bred individual.
Conservation
Mormyrus caballus has not been evaluated by the IUCN Red List. FishBase's own vulnerability index, a modelled estimate rather than a formal conservation assessment, rates the species at moderate fishing vulnerability (40 out of 100).
Its documented range is limited to the lower and middle Congo River, with only an unconfirmed record further south in Angola, which would make it a range-restricted species if that limited distribution holds up under further survey. No population trend or threat data is available in the sources reviewed here, and the appropriate conclusion is that its conservation status is genuinely unknown rather than presumed safe.