Kansai-ben. You've probably heard tons of characters speak it, maybe just without realizing it. First one that comes to mind for me is Ichimaru Gin from Bleach, hopefully you've watched the anime then you'd have heard him speak and you'd hear the difference ^^
An example of Kansai-ben would be as mikkih provided, where the "nai" becomes "hen"
Mieru -> Mienai -> Miehen
Taberu -> Tabenai -> Tabehen
There are nouns too that are specific to Kansai-ben, an example would be "akan" which could probably be translated to "dame" or "shimatta" (oh no/this isn't good). Or "ookini" which is the same as "arigatou" (thank you).
Basically Kansai-ben differs quite greatly from standard Japanese. It's often hard for even native speakers of standard Japanese to understand Kansai-ben, so if you're learning Japanese you should probably ignore it for now and come back to it when you're more proficient

Besides, Kansai-ben is actually a group of dialects including Osaka-ben, Kyoto-ben and many more, with differences between them so yeah, save yourself the trouble and don't confuse yourself unnecessarily 8)
Check out
wiki for more details if you're interested.