Most opponents are unlockable once you defeat them in the course of your game. Some however, are well hidden and must be found first before you can fight them.
Unlock - Ted Tred (Tire World #1)
Walk forward down metal great. Walk forward along tire pile to avoid fighting other characters. Go right at wall, walk on tire pile to tire bridge. Jump on tire bridge and go over the wall to next level. Once you find Ted Tred, defeat him in battle to unlock him.
Unlock - Rangle (Auto World #1)
Walk along car pile on the left side towards wall. Go right and walk on other car pile towards Rangle. Follow car pile to conveyor belt,jump on and walk up beltway to next level. Once you find Rangle, defeat him to unlock him.
Unlock - Big Dig (Secret Level)
Find pipe exit in first level. Go straight along the wall on right side. It leads to two pipe exits. Exit pipe on the right. Fight gas chromie. Go towards the right and walk along the wall on the left that leads to Nailer. Find Nailer, defeat and unlock him. Afterwards, continue going right from Nailer and look for explosive sign. Knock down the sign and pipe wall to enter hidden world and find Big Dig. Defeat him to unlock him.