The name is the only evidence there is, and it will not settle. Aja is a common Sanskrit word for a goat. It is also, read as a-ja, the negation of born, so the unborn. Ekapada is one-footed, or having one foot, or standing on one support. Every combination of those has been proposed. The one-footed goat has been read as a lightning image, the single shaft coming down, with the goat brought in from its role as a sacrificial animal. The one-footed unborn has been read as an abstract or primordial figure, the support on which something rests, which fits a later Sanskrit habit of talking about a single foot as a quarter or a basis.
None of these can be tested, because there is no passage to test them against. This is the point at which the atlas has to be careful about a word. The identification is not debated, in the sense the atlas uses that word: debate means camps exist, holding positions that can be read and weighed against each other. Here there are proposals, offered tentatively, mostly in the nineteenth century, that nobody has since defended at length because there is nothing to defend them with. The status is unknown, and the entry says so, with an open question naming what would actually move it.
The later material does not help, and it is worth saying why. Aja Ekapada turns up in Shaiva contexts as an epithet, which could be a real survival of an old association or could be the tradition doing exactly what a modern reader does when handed an empty name, which is fill it with whatever is nearest.