There are many degrees of rare fish, in light of what's been brought in lately. Some rare fish are rare because they're nearly extinct in the wild, like Neoceratodus fosteri, is rare in the wild due to over fishing and habitat loss.
There are other rare fish like the cluepea in the pictures above, only rare in the hobby, since they're hard to keep and transport, but the desirability is not so great.
Toxotes blythii, is so rare that most people, probably 90% of the people on this board have never even heard of it, its pattern and coloration is certainly not to be overlooked, and worldwide in the hobby there are probably less than a few dozen in the hobby. And we do not know how many are in the wild, but I know that the collection site is very remote, we were going to plan a trip to go there this year, but ended up in China instead.
Scatophagus tetracanthus is rare because of its restricted location, lack of commercial collection and export, but relatively desirable in the hobby.
Cichla marianae "fogo" is rare due to the export country restrictions on export, remote collection area, takes 3 flights just to get there, relatively high in demand but unable to obtain
Leptobotia elongata, rare in the wild, will be extinct in the future, remote collecting site, and few people have it in the hobby.
Sceloropages formosus, asian arowana, is rare in the US only because its CITES appendix I and without possibility of import still. However in Asia, this fish is sold everywhere, thousands sold daily and bred by the hundreds of thousands if not more.
Certain oddball, wild albino specimens, etc., are rare because just like albino humans, how often do you see one?
You guys can name other fish you think is rare, and I'll give you a break down of it, the availability in the wild, in the hobby, in the trade and reason for itt, and you can determine its "rare" factor