Back in 1974, a family of three - Ranger Rick Marshall and his two children, Will and Holly, - were rafting down a river when suddenly a temporal/spatial vortex opened and swallowed them up. They found themselves in an articially created, closed universe, 'The Land Of The Lost'. In this prehistoric-like world, they found not only dinosaurs, but ape-like humanoids and a race of lizard-men called the Sleestak.
For two years they tried to find their way home using these TARDIS-like devices knows as "pylons". Rick Marshall accidentally triggered one of them which sent him back to Earth, only to be replaced by his brother who had "just coincidentally" been out on that same river looking for his brother and his niece and nephew.
(Apparently, there is some sort of seal on this dimension's borders - whatever gets out must be replaced by someone else getting in.)
So in 1976, Ranger Rick raised the alarums about this strange new world and of the threat of the Sleestak. Of course, the reason why you didn't hear other TV series talking about it, dealing with it, or reading about it in the Daily Planet, is because everybody was focused on the Bicentennial Celebration, the upcoming Presidential election, and the homicide investigation of celebrated TV detective Ward Fowler (as seen in "Fade In To Murder", an episode of 'Columbo').
In 1977, 'Land Of The Lost' faded from our TV screens, but anybody fluent in televisiology will tell you that the actions and adventures and life in general as seeon that show kept going in Toobworld (or in this case, a nearby closed universe).
As I mentioned earlier, if someone enters the 'Land Of The Lost', someone else has to leave it to maintain its "harmonious" balance. So it's my belief that when the family of three first entered through that vortex, it may have been three Sleestaks who escaped from it. (Or from some other portal into that world. I know of one other one - to be found in "The Painted Mirror" that was kept in an antiques shop. You can see it for yourself by checking out the exhibits in the 'Night Gallery'.)
Wherever they ended up on Earth, those Sleestaks wreaked havoc and probably killed several humans before they were captured or destroyed. If it happened in the televersion of the United States (pre-Kramerica), then it's likely they were exterminated by the Cigarette Smoking Man and reports of their existence chalked up to some kind of college prank or a movie stunt go wrong. The whole mess would be buried in one of those 'X-Files' folders maintained by FBI Special Agent Arthur Dales.
But the word would have gotten out anyway. After all, the Truth is out there. I'm sure 'The Chronicle' probably ran stories about the three alien lizard men all the time!
So people would have known about the Sleestaks and the rumors would have been verified by the time Holly and Will returned with their Uncle and were reunited with their father. (I don't know how they escaped; I'll leave that to the fanficcers.)
That's a possible scenario as to how the world at large, I say, Toobworld at large, to be exact - pay attention when I'm yammering at you, son! - came to know about the existence of the Sleestaks.
Why do I bring this up now. As a prelude to the splainin as to why airline executive Russ would refer to a Sleestak, a creature from a TV show that should share the same universe as himself, and yet be from some unknown dimension.
During this past Sunday's triumvirate of 'The Loop', Russ expressed concern for his young padiwan, Sam Sullivan by remarking how "Thesis" looked like he had just seen a Sleestak. Now, for us, the joke is in the idea of this 75 year old man making a pop culture reference to a kids' show from the early 1970s. But as I stated, Russ should not have had any awareness that such a world existed unless the Sleestaks actually did escape from that closed universe into ours.
And it just occurred to me that Russ might have had inside info not available to the general public. It could be that one of his company's own airliners made contact with that world. We don't know if "The Odyssey Of Flight 33" ever successfully made it back to the "present" (due to how the episode ended on 'The Twilight Zone'), but it might be that they were eventually able to retrieve some kind of data recordings from the plane.
Whether the three escaped Sleestaks were considered a hoax or actually believed to be real, the general public didn't consider them to be much of a threat apparently. By the year 2007, they were looked upon as an old joke, no longer a danger to Earth. They were an old joke that actually became a joke when Toofer, one of the staff writers on the 'TGS with Tracy Jordan' show (broadcast live from '30 Rock') came up with a sketch idea about a pancake house in the 'Land Of The Lost' called Slee's Stacks.
In order for a sketch like that to work on a nationally televised TV show, the Sleestaks had to be well-known by that point in time.
More than likely an SG team accessed that world via the Stargate and eliminated the Sleestak as a threat to Earth.....
'Land Of The Lost'
'The Twilight Zone'
'The Adventures Of Superman'