So somehow I managed to completely miss that Red Rainbow the name was included in that longer quote from 2003, Carrehz. I don't know how but meh. Sorry about that xD. But it is a lot more detailed than what was there when I first got online.
The information I was remembering was from this, I think:
Spoiler
I heard that these ponies are called Valentine's Day Twins (Earth ponies), but I don't know if each baby pony has her own individual name. They were not a traditioal mail order, as they very only available through a stationary company called Current and never sold through a totally My Little Pony offer.
One of them is white with bright red hair and her symbol is a red heart with a pink and red arch above it. Her name may be Red Rainbow.
The other is purple with pink hair and her symbol is a pink heart with a purple arrow going through it. She has a First Tooth. I don't know what year they were available.
I had thought DV was the source of Red Rainbow but DV didn't give an explanation as to why that name. On other pages DV credited if an individual gave information, so this seems to be something she's remembered or thought was true from her own experience instead.
What's interesting is that a lot more possible info has been added between this and the 2003 later version. I wasn't in the community then, I was on hiatus, but I was told by friends that DV came under fire for not being up to date and for having big gaps in around that time. And she decided then to focus on US stuff, so I guess that's when she did more research. BUT in terms of the name Red Rainbow, it doesn't offer us any more information as to where it came from, instead it adds a few more theoretical names to the puzzle. I guess it doesn't totally matter, it's not like other ponies don't have unofficial names, but I am a detail geek and I'm really curious to see scans of the catalogue that BlushingBlue was talking about ;) and it doesn't seem like DV had that information either.
DV is the backbone for all US-format ID sites, including the Wiki, Reef, etc. So if DV didn't have the info, probably other sites wouldn't have it either unless someone produced it at a later point and added it somehow. That that didn't happen suggests maybe there ISN'T a surviving example of this catalogue..? At least, not in our community. Maybe in a catalogue collecting community. That's where I got most of my argos scans from...