Okay, so, I was trying to remember few cars that are can't be downloaded anymore, as if you try to find those in Google or your favorite web searching engine, 99 of 100% you will get NFSU2 versions of those mods or even versions of them made by different creators (which makes the finding of the car you wanted MUCH harder as most of people don't mod NFSU1 at all), but before you read, be sure if the car that you remember has been lost and no longer available, and you don't have any files of it, you're welcome. However,
if the car is still available on outer source, like, for example, nfsko or playground(dot)ru, it will
NOT be listed here as long as the sites are up until it will be permamently closed (most of Chase_UC's NFSU1 cars from early 201X are the most common examples since they were reuploaded on nfsko, but the author is mentioned as "Alikash"). If the car has been found after being lost since those years, the text will be crossed out
like this.
BMW M3 E36 by Chase_UC: a lost BMW that probably replaced Civic. I probably had this car on some sort of old Acer computer with Windows Vista installed. It was probably hosted on NFSCars, like all Chase_UC's NFS Underground 1 cars, but unlike other Chase_UC's cars, this one was not rehosted by Alikash (the guy who reuploads Chase_UC's and Tails_16v's NFSU1 cars and claims those as his creations).
This video shows it and next two lost cars.Pagani Zonda Cinque by Chase_UC(?): another lost vehicle that was probably made by Chase_UC, much like his aforementioned BMW. I don't really know which car was replaced by this Zonda in NFSU1. And like the lost BMW M3 E36, this Zonda was not rehosted. From Zondas for NFSU1, only Zonda R is online
Ferrari F340 Spider by ? (let me know if you know creator of this F430): a Ferrari that was made before I did one (much like Chase_UC's BMW M3 E36). Again, it was not saved like the aforementioned BMW and Zonda, and that Ferrari was hosted on NFSCars.
Fiat Palio by ? (let me know if you know creator of this Palio): a probably impossible-to-find Palio that replaces Dodge Neon.
It was seen on BelotoPlays' video (a Brazilian YouTuber who solely speaks Portugese, probably to attract Brazilian people), but unfortunately, it is very impossible to find that Palio for NFS Underground 1, as I always get this car as a mod for other games than NFSU1.
Ford Mustang SVT Cobra R by ? (we still don't know who made this, probably Chase_UC again?): This car is seen in (now deleted) AymericTheNightmare's YouTube video, and much like other cars I listed before, this car is not downloadable anymore thanks to NFSCars being redesigned so it accidentally deleted most of NFS Underground 1 mods (they were claimed as viruses, but they have actually harmless tool), and we will never know the car that would be replaced with this Mustang (unless you know it, of course). However, I will make my own Mustang SVT Cobra R as a replacement of this unobtainable Mustang someday in the future.
Mitsubishi Lancer Evolution X by Normo: I still remember good old times from now closed site called Driving Passion (formerly GTRCars) and there was this Lancer that was probably converted from Forza to NFS Underground 1. I still don't know more about this lost car, so yeah, I'll count this car as lost car until it gets found by someone else.
Toyota GT86 and Subaru BRZ by SamuraiSportTeam: They told me at Facebook that they lost files of their Toyota 86 and Subaru BRZ for NFS Underground 1. The screenshots of them are still shown in the internet, but all of the download links to those two are not new or missing, which it means that this mod is now can be counted as this.
BMW M3 E46 by AJ_Lethal: If I recall correctly, this one replaced Supra rather than MX-5, and it also had different customization such as M3 GTR parts as customization options. I looked up on NFSMods - nothing. Looked up on this site (i.e. NFSAddons) - still nothing. Thus this BMW, along with aforementioned M3 E36 (not the one that replaces Neon), are the only BMW cars that are M.I.A. Like Mustang, I will also make my own M3 E46 someday in the future (thanks to LRF for noting this out) (car is still available on creator’s blog, NFS Rides CR, though it just can’t be found by search engines like google)
Chevrolet Corvette ZR1 by Martin the hedgehog (who got later renamed to Sirius-R and now known as S15Fox (at least on DeviantArt)): Was available on NFSCars, but now it's no longer downloadable (it was still listed until the site's closure in November 29, 2023). I might ask the creator of this Corvette if it is lost.
Ferrari 575 GTC by golfstationwagon: What I could find anything about it is by looking at the archives of NFSCars and I found
this. No links available for it for NFSU1, however. If you try to find a working link, then only the broken ones for the same car but for NFSU2 on bond007's site instead.
This is what I could remember from lost NFSU1 cars. If you know more cars that you had but for some reason it's impossible to get it now, let me know.