It doesn't exist that pattern, mainly because there wasn't Dragon slot for four generations, and fighting I already talked about.
The truth is , everybody was used to Ash get regional bird and a trio of water/fire/grass all the time, from Kalos ahead that changed.
I think the difference between out viewpoints is you're only looking at the core lineup while I'm looking at the whole thing and noting that spots get cut when they limit things to Pikachu + Top 5.
Disclaimer: The following is just a theory and not a statement of fact. If you disagree that's okay.
Counting Larvitar it would be more accurate to call the Dragon slot the Pseudo-Legendary Slot at least till we get an official name for that "group" of Pokemon. Notably they've been an every even number generation thing:
2: Larvitar
4: Gible
6: Goodra
8: Dragonite... we've been robbed of Ash getting Dreepy I tell you! Robbed! So okay since Dragonite isn't Galar it might be more accurate to say he fills this spot every odd number series: OS (1-so both Kanto and Johto), DP (3), XY (5), and JN (7)
Unova went up to 9 and was OS 2.0 (so counting Kanto and Johto)
Kanto/Johto Starters = Unova Starters
Pidgeot = Unfezant
Kingler = Palpitoad
Primeape = Scraggy
Donphan = Krookodile
Butterfree = Leavanny
Muk = Boldore, but wait wouldn't Garbodor make more sense here you might say?
Well looking back at Hoenn and ahead to Alola it appears every odd number generation they'll give Ash some sort of "elemental embodiment".
Gen 1: Muk-pile of toxic sludge
Gen 3: Glalie-a ball of snow/ice
Gen 5: Boldore-a rock
Gen 7: Melmetal-a mass of living liquid metal
There is also another odd group of special pokemon based around "events" either in-game or through distributions (gift, trade, overworld encounter, fossil, etc.) that at least at the time of that generation were unique or one of a kind. Lapras, Snorlax, Mr. Mime, Dusk Lycanroc, Solgaleo, Nagnadel, and Dracovish. And there is no limit to these they can give him as many as they want and they can be any type.
The sole oddity among his captures after the end of the core Flying, Grass, Fire, Water team is Gengar, but in hindsight we know now this is Ash's last hurrah, they could be correcting their fumble with Haunter back during OS, they could be wanting to check off all the types for him, and well Gengar is a Kanto Pokemon. First generation always gets favoritism like this.