In which of the MGS series are you able to equip the Stun Knife?
I have only played MGS4. Is the Stun Knife available in any other game in the series?
From the 'Blade Weapons' wikia page:
[...] was used by Solid Snake during Liquid Ocelot's Insurrection in 2014 [...]
So it first appears in MGS4, however...
Upon recruiting Old Snake in Metal Gear Solid: Portable Ops Plus, the stun knife can also be procured.
So it's available in both MGS4 and Portable Ops Plus