If I could bring a pokemon with me into army, I would choose Diglett.

Why?

1. Can help to dig shell scrape, fire trench, MG trench, command trench.
2. Can help to recce.
3. Can set off enemy tripflares at night and screw up their ambush.
4. Can help to clear minefield.
5. Can turn enemy claymores backwards at night.
6. Can summon earthquake and pwn the enemy's firebase.
7. Can overturn enemy combat trains.

Other good pokemon to bring:

Abra: Teleport you everywhere.
Pikachu: Can summon CAT 1.
Espeon: Its clairvoyance would be extremely helpful when people lose stuff.

Bad pokemon to bring:

Magnamite: Screws up your compass.
Hypno: As if everyone isn't sleepy enough already.
Charizard: Not tactical, especially at night.