Elemental Weirds are, as the name implies, an elemental race from the history of Dungeons & Dragons. Uniquely for elementals, they've actually changed quite drastically between Advanced Dungeons & Dragons and Dungeons & Dragons 3rd Edition.
They debuted as the "Elemental-kin: Water Weird"; an intelligent but usually hostile water elemental that could shapeshift between the form of a mass of water and a snake made of solid water. They were pretty much generic cannon fodder enemies. This version of the creature debuted in the original, tournament-dungeon version of The Lost Caverns of Tsojconth for Advanced Dungeons & Dragons 1st edition, and was later included in the first edition Monster Manual, where Ernie Gygax is credited as its creator. It was updated to 2nd edition in the Monstrous Compendium Volume Two, and is reprinted in the Monstrous Manual under the "elemental, water kin" heading.
In Dungeons & Dragons 3rd Edition, however, the Elemental Weird - which debuted in the Monster Manual II - was instead stated to be a species of elementals found all over the Elemental Planes, taking the forms of beautiful women made up of elemental matter, be it Earth, Air, Water or Fire. These creatures are described as elemental oracles, acting as seers and fortune tellers. This version of the Elemental Weird would receive an Ecology of the Elemental Weird article in Dragon Magazine issue #347, which also brought back the original Water Weird by stating that infant Elemental Weirds - statted here as "Lesser Weirds" - spend their lives as snake-like elementals tied to a mass of elemental matter, until eventually they mature and transform into the shapely elemental nymphs. Frostburn would expand the list of four elemental weirds with two new ones from the Paraelemental Plane of Cold; Ice Weirds and Snow Weirds.