So, I was working through an electricity-based dungeon I had written up rather off-the-cuff -- I was the DM, with the party consisting of a human Wiz3 and a wolverine (yes) Rogue3. One of the monsters in the dungeon is a tumblespark that I had straightforwardly backported from the PF SRD to use in this dungeon -- 3.5e itself doesn't stat any lightning elementals I know of.
However, wolverines, of course, like any other predatory animal in D&D, have the standard 'claw, claw, bite' set of natural attacks, and that was what our wolverine-rogue was using against the tumblespark. Should he have taken electricity damage on the tumblespark's attacks of opportunity for attempting to attack a lightning-subtype creature with a natural weapon, or does RAW prefer the interpretation I made at the time, which is that attacking a lightning-subtype creature with a natural weapon draws a standard melee attack of opportunity from that creature, not an energy-based attack of opportunity? Furthermore: does RAW even allow for energy-based AoOs from creatures that would sensibly use them, or does it force AoOs to be of one or more of the three mundane damage types (piercing, slashing, bludgeoning)? And could the tumblespark have burned its Arc ability for the round as an AoO, or must it use it on its own turn?