night exclusive, shit>sentret
day exclusive, shit>ledian/spinkrak
day and night exclusive, version exclusive, shit>mareep
good (but not in crystal)>unon
worse than shit, abysmal>pineco
1% encounter outside of swarm
The problem with the Gen II Pokemon is that they were rare, shit or so late game that they didn't matter. Mareep is one of maybe three lines that is none of those.