Video games from the 70s are like movies from the late-1800s-early-1900s, people thought it was a passing fad, so they didn't really try to preserve them. unlike movies, video games are more future proof, so more games from the 70s exist today. But some did slip through the cracks. 

Oregon (lost original version of "The Oregon Trail" computer game; 1971)

Everyone loves the dysentery inducing, river fording, fort trading, Oregon Trail! But not everyone knows that the Apple 2 version is not the original version. The original version was made in 1971 and was simply called Oregon, it was created by history teacher Don Rawitsch and programmed by math teachers and programmers Bill Heinemann and Paul Dillenberger for the HP-2100 Teletype Device (cool name). Gameplay-wise, it doesn't seem that different from the classic one. After that year's semester, Rawitsch printed out the source code for Oregon and gave one to Heinemann, Dillenberger, and saved one for himself. Rawitsch later joined MECC in 74 and made revised versions of Oregon between 1974 and 1977. You can play a 1975 version and a 1978 version if you do the following:
1. Telnet to mickey.publicvm.com
2. Slowly alternate between CTRL+J and CTRL+M until you see PLEASE LOG IN
3. Enter HEL-T001,HP2000,1
4. Enter exe-oregon for the 75 version or exe-ore2 for the 78 version

Rawitsch lost his copy of the source code, but in 2017, Heinemann said he has his copy but said his son isn't old enough to take stewardship of it (you're like 70 something bill and your son is probably in his 30s, let your son take the family heirloom). a 1975 version was found by Micheal Gemeny and is the oldest version we have so far. Bob Brown created the BBS server where you can play the 75 and 78 version mentioned above.

Laser Clay Shooting System (lost Nintendo 16mm film shooting gallery game; 1973)

In 1973, Nintendo had a great idea, replace all the empty bowling alleys from the 60s with video clay shooting. These were known as the Laser Clay Shooting Systems, where you would use a light gun and shoot video clay pigeons (duh!). This was technically the first video game Nintendo made. After the 1973 Oil Crisis, orders slowed down, but Nintendo made a smaller, cheaper version called the Mini Laser Clay Shooting System, a mini-console before mini consoles, Nintendo sure was revolutionary! all that exists today of LCSS are photos.
 

Fascination (unreleased adult Nintendo arcade game; 1974)

Due to the subject matter, this part is hidden, viewer discretion is advised

EVR Race (lost Nintendo arcade game; 1975)

EVR Race was Nintendo's first non-16mm game, it used EVR film (which is basically the missing link between old film, and VHS) to replicate Horse Betting and Race Betting. No complete cabinet has resurfaced and only the calibration tapes have been found. Judging on the size of the machine, and the type of media it was on, it's unlikely these were repaired often and were probably left to rot.
 

Heavyweight Champ (lost SEGA arcade boxing game; 1976)

I've talked about this, and there's already a thread about this, so I'll keep it short. Heavyweight Champ is a significant game, it may be the first fighting game ever made. the characters kind of look like caricatures of Muhammad Ali and Richard Dunn (not Tim and Eric Richard Dunn) who had a title match in May of 1976. The machine itself had a boxing glove on each side that would be used to control each fighter. There's only one cabinet known to exist today and it's in horrible condition, the bezel is smashed and the insides look like they've been in poor weather conditions. Unfortunately, unless we find a cab, and have a team of people that can replicate the internals (its a discrete logic game), this may stay lost.
 

Pyramid (lost arcade game; 1978)

Pyramid was a Japanese clone of Breakout you have to destroy a bust of King Tutankhamun. from the photos I've seen, it looks nice for 1978, it even has hieroglyphics. I looked at a website that talked about the game and they mentioned MAME, so maybe there's a ROM of the game floating on the internet?

EDIT: While I was watching an episode of the show Gamecenter CX, I noticed that the host actually plays this when he goes to an arcade based in Osaka. So far, that is the only avalible footage of the game in action (as of 7/24/20).

Dracula Hunter (Lost Arcade Game; 1979)

Dracula Hunter is a mysterious game. I did some research on it and almost everyone who said they played it at one point, just can't find a cabinet or a ROM. Some people called it an illusion game! In the game, you play as a priest and you have to defend a sleeping princess from demons, occasionally, the castle doors open and you have to shoot a crucifix beam into the building and it explodes. Besides a reference in a manga, a couple of videos containing footage of the game, and a yahoo auction for the machine that sold for 2.7 million yen, there's no public ROM or cabinet of the game. The whereabouts of the yahoo machine are unknown, but hopefully whoever owns it is trying to dump the game.

Sources:https://lostmediawiki.com/EVR_Race_(lost_Nintendo_arcade_game;_1975), https://lostmediawiki.com/Fascination_(unreleased_adult_Nintendo_arcade_game;_1974), https://www.lostmediawiki.com/Heavyweight_Champ_(lost_SEGA_arcade_boxing_game;_1976), https://www.lostmediawiki.com/Laser_Clay_Shooting_System_(lost_Nintendo_16mm_film_shooting_gallery_game;_1973), https://lostmediawiki.com/Oregon_(lost_original_version_of_"The_Oregon_Trail"_computer_game;_1971), https://lostmediawiki.com/Dracula_Hunter_(lost_arcade_game;_1979), https://lostmediawiki.com/Pyramid_(lost_arcade_game;_1978)