I'm pretty sure it'll evolve after you level it again.
Pachirisu does not evolve :(
Level 10
level 25
at level 20
level 40
It will evolve at level 48
it will evolve at level 21
Arbok does not evolve
Pachirisu does not evolve :(
level them up
If your question implies that you caught a Pokemon that has already passed the minimum evolving level, but has not yet evolved, simply level it one more time, and it should evolve. (However, it CANNOT be level 100 already.)
level 44
Level 10
level 25
at level 20
Level 40.
level 40