Equuleus's own article calls it slight, faint and nearly without lore of its own, the second smallest of Ptolemy's forty-eight. What that thinness obscures is that this little horse's head sits inside one of the oldest thematic groupings in the sky, even though it contributes nothing to the theme by shape.
The stretch of sky running from Aquarius and Capricornus through Piscis Austrinus, Eridanus, Delphinus and the ship Argo before its eighteenth-century division carries a single old reading, water, inherited by Greek astronomers from Babylonian sky-lore because the Sun crossed this region during the ancient Near East's rainy season. Historians of astronomy label the loose cluster the Heavenly Waters, a name used for convenience rather than an ancient formal boundary, since Ptolemy's own forty-eight were sorted only into zodiacal, northern and southern lists.
Equuleus is the odd member: a horse's head has no obvious connection to water at all, and its presence in this neighbourhood looks more like an accident of location, tucked beside the far larger winged horse Pegasus, than a deliberate thematic choice. That makes it a useful check on the whole idea of the Heavenly Waters as a designed set rather than a retrospective label: at least one member was clearly grouped by where it happened to sit in the sky rather than by what it was ever supposed to mean.