While information on the betas of Pokemon is scattered, I have been able to track down several sources of beta information for your perusal! Listed by generation, it is as follows.
In Generation 1, beta content is limited. However, I think you might find these records of beta names for the Kanto Pokemon interesting, as they changed during English localization quite a bit. Here is a list of known promotional names at one point in localization, as well as a promotional poster showcasing some different ones. Worth noting also are the glitch Pokemon manifesting as missingno; there is one each for every gen 2 Pokemon by gen 1 index number since, in early development, gen 1 included gen 2 Pokemon.
Generation 2 has a rich depth of known beta content, the largest depository of which would have to come from the dumped roms of the Gold and Silver prototypes. The Cutting Room Floor has a nice article on this, which includes a subpage on just the Pokemon with comparisons to their currently existing counterparts--and yes, names are included. This provides a very comprehensive look at the G/S fragment of beta history.
Generation 3 has little known info. Some beta designs are known, however.
Generation 4 has few known beta Pokemon, but some beta spritework.
Generation 5: There are no specific beta designs of Pokemon known for gen 5 (as far as I know). One lesser-known tidbit, at least, is that alternate formes for Pokemon receiving them in B2W2 were known about when Black and White were released, with the code of the game showing alternate forme values to reflect this. Data exists also for the moves Freeze Shock and Ice Burn. if the game is manipulated to allow black or white Kyurem to be used in a link battle between BW and B2W2(which cannot ordinarily occur), the forme will use Kyurem's normal sprite on the B/W screen and the moves will work as intended while playing beta animations. I sadly have no footage of this, as I discovered it when I was thirteen or so messing around with an action replay.
Generation 6 showcases data for AZ's Floette, known also as Eternal Flower Floette. This Pokemon, along with its signature move Light of Ruin, was never released for players to battle with and appears only in X and Y cutscenes. The move is never seen.
Generation 7 has few notable beta or unreleased Pokemon aside from Eternal Flower Floette, who is in the code of this generation's games as well.
Also worth noting is Bulbapedia's list of unused Pokemon and character designs. These are some of the more well known ones, but the page is certainly worth a look, as it includes multiple generations an dsome of the more random scrapped designs. (Credit to Stakatacool for posting this first)
If you ever have a craving for some beta knowledge on any game, I suggest you check out The Cutting Room Floor, a site dedicated to cataloging all sorts of facts on the matter. Here is a link to their directory on the Pokemon franchise!
Links, in case the embeds don't load on your device:
(1): https://glitchcity.info/wiki/Early_English_promotional_Pokémon_names
(2): https://i.kym-cdn.com/photos/images/original/000/935/680/96b.jpg
(3): https://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_by_index_number_(Generation_I)
(4): https://tcrf.net/Proto:Pokémon_Gold_and_Silver
(5): https://bulbapedia.bulbagarden.net/wiki/Pokémon_Ruby_and_Sapphire_beta#Pok.C3.A9mon
(6): https://tcrf.net/Pokémon_Diamond_and_Pearl#Unused_Pok.C3.A9mon_Back-Sprites
(7): https://tcrf.net/Pokémon_X_and_Y#Eternal_Flower_Floette_and_Light_of_Ruin
(8): https://bulbapedia.bulbagarden.net/wiki/List_of_unused_Pokémon_and_character_designs
(9): https://tcrf.net/Category:Pokémon_series
