Mimic's go way back to the first edition of Dungeons and Dragons. The first Illustration of the Mimic, was that of it's iconic shape an unassuming chest. I personally think this Mimic version may be a little over powered. It is a good start though.
MimicLevel 7 Elite Lurker
Large Aberrant Beast (Shapechanger)XP 600
| Initiative +8|
Senses Perception +11, all-around vision, darkvision
HP 130; Bloodied 65
AC 23; Fortitude 25, Reflex 24, Will 22
Saving Throws +2
Action Points 1
+12 vs. AC; 2d8+5 damage, and the target is grabbed (until escapes). Checks made to escape the this grab take a −5 penalty.
The mimic can make 2 attacks per round with it's pseudopods. It can form up to 8 pseudopods at one time from it's body.
Each round the mimic holds it's grabbed prey it automatically causes 15 points of damage.
+12 vs. AC; 2d6+5 damage.