Just found a new MI rare early in the game!
Dropped from that big dog in putrid Den in wightmire
Nice find. Too bad the best stats on it (physical resist, % physique) are from the affix. 
Not bad for something that’s been “slightly-chewed” by a bone rat though. 
I guess that rat really likes chewing on bucklers. I got a Flamewall Bernard’s one. What does MI mean?
Monster Infrequent.
As above, Monster Infrequent. Green items that only drop from that mob or that kind of mob. For example, Loxmere’s Frostblade drops from the Loxmere Nightmage hero, and Troll Bonecrushers drop from trolls. In this case, it appears the Putrid Den boss is the only monster who chews on bucklers (specifically those used by prison guard Bernard). 
Double item skill?
Never seen that before 