Just to have them all listed in one post;

TMSB -
https://archive.org/details/TwistedMetalSmallBrawlOct152001prototype
*Only just found TMSB's prototype so I'm not aware of all the differences yet outside of a few minor things.

TM4 -
1 - https://archive.org/details/TwistedMetal4Sep291999prototype
Info:
https://tcrf.net/Proto:Twisted_Metal_4/September_29,_1999_Build
2 - https://archive.org/details/TwistedMetal4Oct211999prototype
Info:
https://tcrf.net/Proto:Twisted_Metal_4/October_21,_1999_Build
3 - https://youtu.be/UvTwBJfMC00 Video of earlier TM4 prototype

TM Black -
https://archive.org/details/TwistedMetalBlackApr132001prototype
https://archive.org/details/TwistedMetalBlack20010412/Twisted_Metal-Black-2001-0
4-12-Game-21.PNG
*both links are the same April 2001 prototype. cutscenes were unfinished, warthog was named white knight (or W-Knight in-game) along with a different driver name; the stage images were different, etc. etc.

Rogue Trip Vacation 2012:
https://archive.org/details/RogueTripVacation2012Aug301998prototype

TMBO (Europe) Beta:
https://archive.org/details/TwistedMetalBlackOnlineEuropeBeta

TMHC Prototype:
https://archive.org/details/TwistedMetalHarborCity20040129/Twisted_Metal-Harbor_
City-2004-01-29-Game-05.PNG

TM 1995 Beta:
https://archive.org/details/TwistedMetal19950817/Twisted_Metal-1995-08-17-Game-0
4.PNG

TM3 Prototype:
Videos:
1 - https://archive.org/details/012136
*This commercial shows a very different looking Washington, DC.
2 - https://youtu.be/epZZZSjvejc
*PlaystationMuseum's video of TM3:Calypso's Revenge Prototype
3 - More beta footage seen in the TM3 Video Demo/Trailer showing a different looking Egypt.

TM2 World Tour Beta:
1 - Link for game at CoolROM's site.
2 - Video of earlier prototype seen in Twisted Metal 2 Video Demo/Trailer showing different vehicle names (DeadHead instead of Hammerhead, Bam Bam instead of Mr. Slam). This video was in Interactive CD Sampler Pack Volume Three.
-Additional beta footage in alternate trailer - https://archive.org/details/012505

TM Head-On / World Tour (PS2):
https://archive.org/details/TwistedMetalHeadOn20040129
TM Head-On / Rage (PS2)
https://archive.org/details/TwistedMetalHeadOn20041101

And of course the early versions of TM PS3 can be seen in old videos of the game pre-release (E3 2010, G4 videos).