Are gehreleths mentioned at all in any of the 3rd edition monster manuals? I can't seem to find them around. If not, are they given in 3ed stats anywhere unofficially?
'leths
Why is it that they 1) made up new names for the fiends in the Planescape setting, and 2) changed the names back for the 3rd edition?
1. They didn't. The new names were invented several years before Planescape came out, for the Outer Planes Appendix. Terrified of angry fundamentalist mothers, TSR dropped the word "demon" or anything like it. Planescape inherited the new names.
2. Wizards of the Coast wasn't afraid of angry fundamentalist mothers, so they put the old names back. They actually started doing this in very late, WotC-era 2nd edition. Note that Tales From the Infinite Staircase uses the word "demon" once (there's a glabrezu in the adventure who's sick of being a clever Planescape-style tempter and just wants to be a mindless 1e-style killing machine, so he calls himself a demon instead of a tanar'ri).
Interesting. What was the idea behind changing the original names of the planes?
That was all Planescape, and specifically David "Zeb" Cook.
I'm not sure what he was aiming for there. I guess he just liked the new names better. In some cases (particularly Mechanus and the Beastlands instead of Nirvana and the Happy Hunting Grounds), I do too. Since it was established that the devils were baatezu, it made sense to make the Nine Hells Baator. The Outlands is easier to say than Concordant Opposition (which is called "a mouthful" in the Planescape Campaign Setting box). Carceri is a reference to the art style Cook was shooting for, as well as (of course) being Italian for prison.
Maybe Ysgard is supposed to be a combination of Asgard and Yggdrasil. I'm not sure.
I've always preferred "the Outlands" to the title "Concordant Opposition". It seems to connect much better with the notion of a "planescape".
They (being the farastu, kelubar, and the shator) are detailed fully in the 3e Fiend Folio were they are titled 'Demodands'.
I'm certain there are 'unofficial' 3e conversions here and there... and mostly likely in the EN World creature catalog, but I don't have the time at the moment to find the appropriate links...