The lamia

[Rocl]

It's likely that beings like Echidna are actually voidsent, while Lamia / Nagas are "artificial voidsent" created by the Allagans (this is supported by museum displays in the Fractal Continuum for the Naga). The ones in Satasha (Hard) are also probably Allagan creations--escaped from nearby Dalamud debris-- the classification of them as "Drowned" being a case of mistaken identity due to general ignorance of Eorzeans.