EXCLUSIVE

One of Migos' most iconic songs is a straight cover -- at best -- or an absolute ripoff at worst ... according to another rapper who's suing the trio.

Leander C. Pickett -- better known, he says, as M.O.S., -- says he recorded a song called "Walk It Like I Talk It" way back in 2007, and released it on a DJ Folk mixtape in 2008.

In docs obtained by TMZ ... Pickett says he realized, in January 2018, Migos had released a track on their "Culture II" album with an amazingly similar title -- "Walk It Talk It" -- which uses a "substantial portion" of his version.

Play video content Migos, M.O.S.

It's true, when you listen to both songs ... it's hard NOT to hear the similarities, especially in the chorus to the songs. For what it's worth, Pickett says he filed for a copyright on his song in March 2018.