Maniac Tunnel: 2 species in 2 games

Maniac Tunnel (26+ Different Unown Caught): 2 species across 3 versions

Diamond and Pearl: 2 species, all by walking in tall grass or a cave

Species	Levels	Chance	Game page
Geodude	24	80%	All places
Hippopotas	22 to 25	20%	All places

Platinum: 2 species, all by walking in tall grass or a cave

Species	Levels	Chance	Game page
Geodude	23 to 25	80%	All places
Hippopotas	24 to 26	20%	All places