I did my research well.
 
 
The official MLP G4 Japanese voice cast to the One Punch Man anime voice cast:  
Miyuki Sawashiro as Mosquito Girl of episode 2 (only)  
Yūichi Nakamura as Mumen Rider / Licenseless Rider [Class C, Rank 1]  
The veteran Wataru Takagi (not the Detective Conan / Case Closed character) as Hammerhead, leader of the Paradisers gang  
Mamoru Miyano as Ikemen Kamen (Handsomely Masked) Amai Mask [Class A, Rank 1]  
The godlike Tesshō Genda as Metal Knight (shown in his robotic drone) [Class S, Rank 6]  
The veteran Minami Takayama as Child Emperor [Class S, Rank 5]  
The veteran Nobuo Tobita as Sitch of the Hero Association  
Yōji Ueda as Drive Knight (or Engine Knight) [Class S, Rank 9], and some minor characters
 

 
Copyrights and trademarks:  
My Little Pony Friendship Is Magic (& Equestria Girls) <- Hasbro, Studio DHX, TopDraw Animation  
One Punch Man <- Yūsuke Murata, Studio Madhouse
 
 
MERRY CHRISTMAS!