Bumidek

Thank you @theSLAYER that explaines it, I missed that.

One more question if I can:
Move Agility on lvl 50 legendary birds Articuno, Zapdos and Moltres does not evoke illegality issue. How is that possible when they learn it at lvl 55 in gen1 and in gen2 they are uncatchable?

 

Mimic is TM in gen 1 and gyarados CAN learn it. It is even famous for creating shiny ditto in gen 1.

https://serebii.net/pokedex-gs/130.shtml
https://bulbapedia.bulbagarden.net/wiki/Gyarados_(Pokémon)/Generation_I_learnset#By_TM.2FHM

Gyarados with move Mimic is marked as illegal move in gen2 games.

With the trade for dugtrio in yellow (wrong sprite in the table btw) it can be possible to trade that dugtrio to crystal and recieve level 5 magneton from trade in power plant.
Rhydon trade in yellow can be level 10 with trading golduck from GSC,

Found another one. There is trade in crystal: get Magneton for Dugtrio.
The table mentions Dugtrio wild. However there is another trade in yellow: get Dugtrio for Lickitung. Lickitung can be breed to level 5.
So with trade in yelllow, trade to crystal and trade in power plant it would be possible to get Magneton at lvl 5.

Also get Parasect for tangela trade in yellow can be level 5 from breeding tangela.

In G/S/C it is possible to catch Dragonair at lowest level of 10 on route 45 with Super Rod. When this Dragonair was traded in Blackthorn city I recieved Rhydon that is marked as ilegal for some reason. Says "invalid: Ingame trade OT has been altered", but it was not. File attached.

In crystal the trade is changed to fDragonair for Dodrio. OT trainer and DVs are the same. I modified Rhydon into Dodrio with proper movepool but it was marked as ilegal as well.

I recreated some other in-game trade pkmn in PKHex and they passed the legality check.

112 - DON - 4DD6.pk2