It's there, there's just no encounter slot for a level 25 variant.
https://www.smogon.com/forums/threads/headbutting-safari-zone-great-marsh-rng-abuse-guide.3460739/
encounter slots 6 and 7 are replaced by the Binocular Pokémon of the Day.
Data from Pearl:
Unless the community knowledge for this is wrong, slots 6&7 are the same level (not 21-26).