You can only fight NPCs that are on the same planet as you. There are some NPCs in space that you can fight while traveling.

Beast (all planet except for Old Vegeta)

Power Level - 100

MP - 150

Moves - Punch, Kick, Uppercut, Jumpkick, Mouth Blast, Tsu u Beam

Dino (all planets except for Yardrat)

Power Level - 300

MP - 350

Moves - Punch, Kick, Uppercut, Jumpkick, Tsu u Beam, Mouth Blast, Laser Eyes

Weak Tenkaichi Fighter (only Earth)

Power Level - 350

MP - 400

Moves - Punch, Kick, Upper Cut, Jump kick, Mouth Blast, Tsu u Beam

Money - $50

Strong Tenkaichi Fighter (only Earth)

Power Level - 450

MP - 500

Moves - Punch, Kick, Upper Cut, Jump kick, Mouth Blast, Fire Kick, Dragon Fist

Money - $100

Renegade Namek (only Namek)

Power Level - 450

MP - 350

Moves - Uppercut, Jump Kick, Special Beam Cannon, Tsu u Beam, Arm Growth

Items - Namek Root

Frieza Henchman (only Namek)

Power Level - 800

MP - 800

Moves - Punch, Kick, Jump Kick, Upper Cut, Death Ball, Dragon Kick,

Money - $150

Saiyan Spirits (only Old Vegeta)

Power Level - 900

MP - 950

Moves - Punch, Kick, Upper Cut, Jump Kick, Saiyan Blast, Gallic Gun, Big Bang Attack

Reward -you get 30MP at the end of the battle rather than only 25. You can only fight one per battle.

Saiyan Rebels (only New Vegeta)

Power Level - 850

MP - 900

Moves - Punch, Kick, Upper Cut, Jump Kick, Death Ball, Saiyan Blast, Laser eyes

Money - $200

Evil Yardrats (only Yardrat)

Power Level - 500

MP - 600

Moves - Punch, Kick, Upper Cut, Jump Kick, Yardrat Combo, Dragon Fist, Yardrat Blast,

Money - $100

Weak Space Pirates: (Can only fight once per travel, but can fight a huge group, if you are strong enough)

Power Level - 850

MP - 950

Moves - Punch, Kick, Upper Cut, Jump Kick, Mouth Blast, Super Strike, Tsu u Beam

Money - $100

Strong Space Pirates: (Can only fight once per travel, but can fight a huge group, if you are strong enough)

Power Level--1700

MP--1250

Moves--Punch, Kick, Upper Cut, Jump Kick, Mouth Blast, Super Strike, Tsu u Beam, Rocket Punch

Money--$250

Sandora Soldier:

Power Level--1800

MP--1800

Moves--Punch, kick, yama lock, mouth blast, special beam cannon, hell flare, 4-way kamehameha, spirit bomb

Reward--$350

Hellena Soldier: (can do 3 attacks per turn)

Power Level--4500

MP--4500

Moves--Punch, kick, uppercut, Special beam cannon, yardrat blast, hell flare, death ball

Reward--$400

SOLDIER 1st Class:

Power Level--1750

MP--2000

Moves--punch, kick, uppercut, jump kick, Kamehameha, Hell Flare, Spirit Bomb, Time Freeze, Masenko

Reward--$300