Missingno. is in the mod since v0.3, and is currently in consideration for where to spawn, the Nether is an option. 1) Another option is to put near Cinnibar Island, to right, under ground. Where it is in the actual game. I suggest putting underground so it doesn't escape and wander off.
pokedex[]
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clockwise from left: Normal, Ghost, Aerodactyl, Yellow, Kabutops