Help with building a race for 3.5 encounter

[Johnngrimm]

I was wondering how people would go about making a bipedal medium sized Displacer Beast.

I want to throw my current party for a loop with a character they never fought before because one of then is metagamming more than I would like even after talking with him. The encounter is going to be for eight characters of level 12 and I could use some help making the main boss of this fight.

I would like this boss to be a bipedal Medium sized displacer beast, with the encounter as a whole taking place in a lowly lit cavern with varied terrain for everyone to fight on.

[Admiral Squish]

Well, the easiest way to do it would just be to keep all the same stats, and just say it's upright, with four weapon-wielding arms. Increase intelligence, add class levels as necessary.