First in Gen 1, Mew was put in as just an easter egg. Supposedly the only members of the dev team that even knew it was in the game data were Tajiri and Morimoto.
1998 - Anime makes its first movie about Mewtwo because of course it does, and also features Mew for the obvious connection and by this point Mew is officially revealed and has giveaways for it.
1999- Anime has proven successful enough that it's still going indefinitely now and is getting movies every Summer. Having the 2nd movie just star the bird trio wouldn't be good enough, so the anime team asks Game Freak to come up with a new legendary Pokemon for them. Sugimori designs Lugia, and movie writer Shudo comes up with Lugia's lore, and Game Freak decides to put Lugia in GS as the mascot for Silver version while Ho-Oh is the mascot for Gold (originally they just planned for Ho-Oh to be the only "uber" legendary in GS like Mewtwo was for Gen 1). Gold/Silver comes out only a few months later that year (almost all Pokemon games from here on come out in the Fall) so this starts the pattern of the final movie of a gen starring a next gen Pokemon to hype up the next game.
In Gen 2 they decided to put in Celebi as a secret Pokemon to copy what happened with Mew. Originally they planned to feature it in the anime as part of the story arc with the GS Ball.
2000 - They aren't messing with Celebi yet, so the anime team picks out another legendary to be the featured Pokemon, so they pick Entei arbitrarily.
2001 - By this point they decided to drop the whole GS Ball storyline in the anime without conclusion and feature Celebi in a movie instead. Suicune also cameos because Crystal is the latest game. Celebi is officially revealed and has a giveaway to copies of Crystal (which still features the GS Ball as part of the Japan-only event).
2002 - Ruby/Sapphire is coming up, so they feature Latias and Latios to hype it up.
With Gen 3 they seem to have decided to fully plan out having another mythical Pokemon in the game data to have Pokemon to feature in the movies and have tie-in events for for multiple years, so they make two mythicals with Jirachi and Deoxys.
2003 - Jirachi focused movie, with Jirachi being officially revealed and given away for the first time.
2004 - Deoxys focused movie, with it being revealed and given away to copies of FRLG (where it first gets alt forms which are shown off in the movie). Emerald is also coming in a few months so Rayquaza also makes a big appearance and Emerald also gets its own Deoxys event and form.
2005 - Diamond/Pearl was originally supposed to come out this year, so this year's movie starred Lucario to hype it up. There was also Mew as a major part of the movie, probably picked tie-in with FRLG, and so they had that year's movie tie-in giveaway be Mew with Emerald having a special in-game event for it.
2006 - Oops, DP got delayed a year, so this year's movie is also hyping Gen 4. It stars Manaphy, which was just some design Sugimori had laying around that they weren't sure what they were going to do with yet, but the anime staff asked for a new legendary to feature so Sugimori picked it out and handed it to them. They nothing more to givaway to mainline games, so they did the event with Pokemon Ranger where you complete the Manaphy mission in it and you get an egg you can transfer to DP once it came out a few months later.
They made sure to have enough mythicals in Gen 4, so in addition to Manaphy and Phione, they made three more with Darkrai, Shaymin, and Arceus.
2007 - Darkrai focused movie, gets revealed and given out for the first time. The movie also features Dialga and Palkia, starting a pattern for the next couple gens where the first movie would also feature that generation's box legendaries.
2008 - Shaymin focused movie, and also Giratina since Platinum came out that year. To give extra incentive to get Shaymin, they put the new Sky Forme in Platinum which is also shown off in the movie.
2009 - Arceus focused movie. Since HGSS was coming out just a couple months later they ended up skipping out doing the full Azure Flute event for copies of DPP and instead focused things the Arceus giveaway letting you access the Sinjoh Ruins event in HGSS. They also stuck the spiky-eared Pichu in the movie, which was another event Pokemon for copies of HGSS.
2010 - Stars Zorua and Zoroark to hype up Black/White coming up, but also feature Celebi and cameos from the beast trio so they had Pokemon to giveaway. Tie-in event was a Celebi and shiny beast trio who could transfer into BW later to catch Zorua and Zoroark in that game.
For Gen 5 they made four mythicals, Victini, Keldeo, Meloetta, and Genosect. Only three got movies. I'm not sure if they made an extra in case the generation lasted four years, or if they didn't expect the anime team to make the first movie about Victini since it was given away soon after BW came out as a timed event to get people to buy the game at release.
2011 - Movie focuses on Victini, Reshiram, and Zekrom. Victini was already revealed and given away at BW's release, but I think they had another giveaway with the movie along other Pokemon.
2012 - Keldeo stars in the movie, gets revealed and given away to go along with it. Kyurem and its new forms are also a major part of the movie because B2W2 came out the same year. They gave Keldeo a new form in B2W2 that also gets shown off in the movie. Meloetta never starred in a movie, instead it got featured in a couple anime episodes and in the short that ran with the movie, and was also given away this year.
2013 - Genosect is left, so obviously it's the featured Pokemon and is given away. But XY was coming out soon, so it also features Mega Mewtwo Y to hype up Gen 6, though the concept of Megas wasn't actually revealed yet so Mewtwo's new form goes unnamed and unexplained in the movie.
Gen 6 was likely going to be three years long, so they made three mythicals. Diancie, Hoopa, and Volcanion.
2014 - Diancie movie with Xerneas and Yveltal. Diance is revealed and given away. ORAS also came out this year a few months later, so they feature a new Mega Diancie in the movie and have an even in ORAS where you get the mega stone when you transfer Diancie into it.
2015 - Hoopa movie with giveaway, and they did the same trick of introducing a new form with Unbound form in ORAS. Since ORAS has every legendary in it that wasn't in XY, they also stuck a whole bunch of legendaries in the movie too. And ORAS has most of the legendaries coming out of what looks like Hoopa's rings, so they seemed to have had this tie-in planned from the start.
2016- Volcanion is revealed and giveaway with the movie, but Sun/Moon is also coming out soon so the movie also features a new Pokemon you'll be able to get in SM with Magearna to hype up Gen 7. Also the movie kind of features Zygarde (it doesn't do much, but they showed it off a bunch in trailers) because had they made Z it would have come out last year, but they didn't and mainly Zygarde got shown off in the anime.
With Gen 7 it seems Game Freak had decided on introducing new Pokemon mid-generation now, so other than Magearna they only stuck a single mythical in Sun/Moon to start, Marshadow.
2017 - The movie this year was that weird AU retread of the first season featuring Ho-Oh. But they also awkwardly stuck Marshadow in there so they could tie-in with giving it away that year. Then USUM comes out later this year, so they put Zeraora in it for next year's movie.
2018 - Features Lugia to go along with last year's movie I guess, but mainly features Zeraora, which is revealed and given away to copies of USUM.
Then Let's Go comes out later this year, so they gave that its own new mythical with Meltan/Melmeltal. They don't make a movie about it though, instead featuring it in the anime series by giving Ash one.
2019 - The movie this year was a 3DCG remake of the first movie, for some reason. I guess you could say it ties into the newest game at the time Let's Go being a Gen 1 remake.
And then in Gen 8 they put a single mythical into the game to start, Zarude.
2020 - Zarude focused movie, gets given away.
And then the anime studio stopped making movies after that, so Game Freak stopped making any more event-exclusive Pokemon since there was no longer anything to tie them into. Though I guess you could say Pecharunt feels like one.
