so here's the real question:
We know that Doomguy/Doomslayer/Commander Keen (and John Wolfenstein, but that doesn't matter) are all related, right?
So clearly they take place in the same universe.

but in the Doom games you go to Mars and it seems it's a pretty red airless cold planet, much like it is in reality.
It's just got some Fancy Future Buildings on it because we're colonizing it. Until the Demons show up, there's not much wildlife, just some humans.

but when Commander Keen goes to Mars, it's still a red (well, pinkish) planet, yes...

But now there are Yorps all over the place!

now the bad guys in the Commander Keen 1 series are the Vorticons, who are from the planet Vorticon VI.
So one solution you might suggest is that all the life you see on Keen!Mars was brought there by the Vorticons, and after Keen defeated them, they all left.

BUT NO!
The story of Keen 1 confirms that the Yorps and the Gargs are native Mars life.

So we know Keen takes place before Doom: Commander Keen is actually the grandfather of the Doom Guy.
And given that he's 8 years old at the time of Keen 1, he probably doesn't have any kids, let alone any grandkids.

So apparently something happened horribly to the native Yorp & Garg population between when Keen took place and when Doom took place.

Hopefully there's at least a big reservation of them somewhere on the other side of Mars from where the UAC set up and they aren't extinct.

Doom 3 makes this even more confusing. I'm not sure how much the Doom the Doom (2016) and Doom Eternal retcons the Doom 3 backstory, but according to Doom 3 there was an ancient martian civilization who ended up getting wiped out by demons... probably.

at least one character in Doom 3 things they used the teleportation technology to go to Earth, and were the early ancestors of humanity.

which makes the question of Yorps and Gargs even more confusing!
If there were Ancient Martians who were related to humans, are humans distantly related to Yorps/Gargs?

BUT IT GETS WEIRDER

Yes everyone agrees Keen 1 and Doom (and Wolfenstein) are related, but why just those three?

because there's at least one more game that takes place on Mars... and features Yorps!

correction, Gargs.
and that's Monuments of Mars!
See that two-eyed monster over on the right?

Looks a lot like the Gargs of Keen 1, right?

but that might just be a coincidence. I mean, how many ways to designed platformer aliens are there? and take place on mars?

EXCEPT LOOK WHO SHOWS UP ON THE HIGH SCORES LIST OF MONUMENTS OF MARS:
Commander Keen!

and Scott Miller, who founded Apogee.
He's also the one who convinced the Carmack/Romero & Tom Hall to make their own games as id Software, starting with... Commander Keen!

But we can make Monuments of Mars work into the Doom/Keen timeline. Probably.

so (spoilers for a 30 year old game), at the end of Monuments of Mars, you find out that aliens had built the Monuments of Mars as an installation to study earth, but abandoned it.

And their left-on automatic security systems are what has been capturing NASA astronauts.
Whoops.

This explains all the hostile robots wandering around the Monuments of Mars, but the Gorgs are apparently biological.

it's always possible the aliens used both biological and mechanical security mechanisms, or... maybe the Gargs aren't part of the security system? Maybe they're just living in the abandoned alien base?

like if you find a building that hasn't been occupied in a decade and when you crowbar the door open, there's an angry raccoon and a couple wasp nests, you don't go "AH! THE SECURITY SYSTEM IS STILL ON!"

So if the Gargs are just wildlife living in the disused monuments, there's (at least) two possibilities:
1. they're native Martian life
2. they're related to the aliens

And I think #2 makes more sense, given the Ancient Origin stuff in Doom 3.

Here's my theory:
The monument-builder aliens came to mars at some point in the past, bringing along some Gargs. Maybe they're the Monument-Builder equivalent of a cat or dog.

eventually for some unknown reason, they leave the earth-studying facilities they'd built, leaving some Gargs behind, and leaving the security systems on.

As for why they left... Maybe they did exactly what the UAC did in Doom 3:
They discovered the ancient cities, found their teleportation tech, started playing with it, and WHOOPS DEMONS

After they'd abandoned the planet, NASA sends some astronauts to explore the monuments. They get captured by the security system, occasionally attacked by wild Gargs, and eventually one unnamed astronaut discovers the truth and frees their comrades.

So both Keen 1 and Monuments of Mars take place in a vague year.
We can guess at Keen 1 taking place around the 80s/90s because of who his grandfather is (John "BJ" Wolfenstein), but Monuments of Mars is "sometime after 1958" (because NASA exists)

So my theory on that one is that Monuments of Mars takes place AFTER keen.
Two simple reasons:
1. Commander Keen left his high score in the game. Ignoring questions of how in-canon a high score table is...

2. NASA couldn't have landed several astronauts on Mars in the 80s or 90s. So let's assume that's a future mission.

So the full timeline:
Long ago: Ancient martians build a civilization, invent teleportation, get into a conflict with demons, flee to Earth and become ancestors of humanity. Mars has no Gargs.

Less Long Ago: Monument-Builders go to mars to study humanity, bring their pet Gargs, build monuments, discover the Ancient's teleportation stuff, get into war with demons, and abandon the monuments, leaving Gargs behind

Sometime in the 80s or 90s:
Vorticons come to Mars.
Commander Keen builds a spaceship and flies to Mars, and sees Yorps and Gargs, plus dangerous security robots (SOUND FAMILIAR?) and the non-native Vorticons

Keen eventually leaves Mars, defeats the Vorticon Mothership which is about to blow up the Earth, and then goes off to their home planet of Vorticon VI to stop the Grand Intellect.

Sometime in the future:
NASA starts ending missions to Mars, presumably unaware of Keen's exploits on the planet. The first astronauts are captured while exploring the Monuments of Mars, because the Monument Builders left their security systems on

Their "last hope" astronaut (John "Last Hope" Monuments Of Mars) goes to rescue them, discovers that this has all been automatic security systems, fights wild Gargs, frees the astronauts.

Even Later (but around 2104 according to Doom 3):
Exploration of Mars involves the UAC, and they discover the ancient Martians, and start playing with their teleportation stuff

A bit later than that (2145 according to Doom 3):
The Doom Guy goes to Mars. It doesn't seem to have any Yorps or Gargs on it. UAC experiments with teleportation Go Wrong and Hell invades, Doomguy rips and tears until they are all defeated.

So between "Sometime in the future" of Monuments of Mars and the "Even Later" of the Doom (/Doom 3) backstory, something happened to the Gargs and Yorps.

Here's my guess:
They, not being native to Mars either, were only surviving because of the Monuments of Mars, which presumable was set up to ensure a proper environment for the Monument-Builders.

Since the Monument Builders are from the same planet as the Yorps & Gargs, they share common atmosphere and food requirements.
But at the end of Monuments of Mars, John Monuments Of Mars turns off their security systems, in order to rescue the NASA astronauts

Turning this off rapidly made the monuments no longer habitable for the Yorps and Gargs. We know they can live on Earth (Keen 2 and Keen 4 show that Keen has a Pet Yorp named Spot)

So my theory is that NASA brought a bunch of them back to earth after Monuments of Mars. They may be all in zoos or special wildlife preserves, or they may be in the wild now or kept as pets, but all the Yorps and Gargs aren't on Mars anymore, because they moved to Earth.

That's all the ones that were on Mars or Earth, I mean.
Possibly the Monument Builders still have some on their home planet.

So yeah. This'll explain why Mars is Yorpsless (and Gargless) by the time of Doom.
The only native Mars life we know about is the Ancients, who are possibly human ancestors.

the only problem with this hypothetical timeline and fan-wankery to tie these games together properly... is that it directly contradicts the story of Keen 1, which calls the Gargs and Yorps "Martians".
Dang it.

it also says they built the robots, which ruins my theory that the robots are from the Monument Builders

Anyway, the next Commander Keen game, the Commander Keen The Commander Keen Mobile Open Bracket 2020 Close Bracket one, seems to have been soft-canceled but if it ever comes out, hopefully it'll finally explain all these important questions.

Hopefully now that the rights to the id Software games have been reshuffled yet again as they're now owned by BethesdaMicrosoft, Tom Hall will finally get a chance to make Keen 7: The Universe Is Toast, and it'll mainly be about how the Keen-Doom-Monuments of Mars timeline works

Here's another question, which I promise not to go into:
Commander Keen is the grandson of John Wolfenstein, right?
Now, that fact was established way back in 1992-1993, in the Wolf3D hintbook.

However in 2014 there was Wolfenstein: The New Order, which is a sequel to Wolfenstein the Wolfenstein Open Bracket Two Thousand And Nine Close Bracket, and it has John Wolfenstein waking up in the 1960s... to discover that while he was in a coma, the Nazis won the war.