Games prior to Ruby and Sapphire not included because you can't transfer Pokemon from them to generation III
Pokemon Ruby Version - 2003
Pokemon Sapphire Version - 2003
Pokémon Channel - 2003 (Jirachi)
Pokemon Fire Red Version - 2004
Pokemon Leaf Green Version - 2004
Pokemon Colosseum - 2004
Pokémon Colosseum Bonus Disc - 2004 (Jirachi and Celebi)
Pokemon Emerald Version - 2005
Pokemon XD: Gale of Darkness - 2005
Pokemon Ranger - 2006 (Mannaphy)
Pokemon Diamond Version - 2007
Pokemon Pearl Version - 2007
Pokemon Platinum Version - 2008
Pokemon Heart Gold Version - 2010
Pokemon Soul Silver Version -2010
Pokemon White Version - 2011
Pokemon Black Version - 2011
Pokemon White Version 2 - 2012
Pokemon Black Version 2 - 2012
Pokemon X Version - 2013
Pokemon Y Version -2013
Does anyone now which games you must have in order to catch them all? It seems far fetched that games that are now a decade old are needed in order to catch them all, but I stopped playing pokemon once I hit high school because I got to busy with school work. The fact that Pokemon Ruby Version and Pokemon Sapphire Version are compatible with games that came out a decade later makes a remake of them a bit pointless, but that's a side note. I'm not really sure if you need all of them to catch them all, but would imagine someone on here knows.
Does anyone know?
Well, actually Ruby/saphire/emerald/fire red/leaf green aren't directly compatible with the games that came out a decade later, they're apparently compatible with the gen 4 games (platinum, Diamond, pearl and whatever other ones came out at that time Heart Gold? Soul Silver? Not sure) and then only via the DS because it had a slot for GBA games as well as DS games when it first came out, and something called the Pal Park or whatever. (This is why I stopped playing pokemon, because I heard rumours that you can send your gen 3 pokemon over but all your IVs get reset, and I worked too hard for that to happen) The gen 5 games then need to get those gen 3 pokemon from the gen 4 games only (so there was no direct interaction between gen 3 and gen 5) and now with pokebank, you still could technically GET those same gen 3 pokemon but only from gen 5 games (so you would have to trade a pokemon from emerald, to Platinum, to Black 2 then to X, for example, in order to get it to the current generation)
Now, having said all that, apparently there's only about 80 pokemon that aren't natively available in X and Y, and while there's a lot of exceptions, most of the unobtainable pokemon seem to be starters and legendaries. (there's a lot more pokemon on the list than 80, but a lot of them are parts of evolutionary lines so I count them as 1 pokemon)
https://www.serebii.net/xy/unobtainable.shtml
so a remake would be quite welcome, however, i do agree that there really wouldn't be THAT many pokemon that would even warrant needing a remake because there aren't that many non legendaries or starters on the list. Really, a Pokemon Colloseum or Gale of Darkness like game would be enough to fill out the rest of the Pokedex if the Serebii list is accurate.