While this is about the OVA, this still contains some sort of spoilers for the anime and manga. Read at your own risk!
Rayearth (jp. レイアース) is an OVA adaptation the CLAMP manga, Magic Knight Rayearth. The OVA consists of three episodes, and deviates quite a lot from the original manga and following anime. The OVA was released in 1997.
While this OVA is known by fans, the relative obscurity of MKR makes this even harder to come across. I haven't seen many reviews of this myself, though I would imagine that it would give mixed reviews due to the divergence from the original canon.
Hikari, Umi, and Fuu are three best friends who are about to finish junior high, but don't want to be separated after moving to different schools. Together they make a wish under a cherry blossom to be together forever—soon after, the three of them keep spotting a strange rabbit-like creature named Mokona. As the girls continue to spot and chase after him, time seems to stop—Tokyo is frozen, silent, and the girls are the only three left in the city.
A strange man from another world comes to greet them. His homeworld, Cephiro, has attempted to merge with the real world in order to keep existing. However, doing this will erase life on Earth as they know it—it's up to them to awaken the hidden Mashin on their planet and save Earth—and Cephiro along with it.
The first difference many will spot is that the three girls all go to the same school this time around. In the original story, the three of them don't know each other, and simply happened to be on a field trip to the same place at the same time through different schools. Additionally, the entire story takes place in the world of Cephiro, unlike the OVA where it takes place in the girls' homeland.
All characters have been given different designs, and the colours of the OVA are a lot more dank and dark than both the anime and original manga. Additionally, some characters have been reworked or changed—for instance, Ferio is not Emeraude's brother, but rather Eagle Vision is. Both characters are also completely antagonists, however Ferio seems to have a bit more of a heart than the one who took his previous role. I do not recall Zagato's purpose in the OVA, since Eagle Vision is the mastermind behind everything. Guru Clef is the most consistent with the original story, as he's still fully on the side of the protagonists. However, his design makes him look a lot less like a baby, and he does a little more than metaphorically push the girls into the deep end.
Regardless of how you may feel about the creative liberties taken, Rayearth is an interesting watch. I personally feel it's better to watch it after having read the manga, since that way you can really appreciate the differences between the two stories.