That depends on what you want the lizard to do.
It could have a lightning damage shield that strikes melee attacker when they damage it. This could just be a permanent supernatural effect set on her (with proper non stacking failsafe) when it spawns and on perception (just for double checking, should the effect wear off because of any particular reason).
Extra electrical damage could be added on the creature’s natural weapon properties obviously.
Another idea would be a lightning breath, this could just be assigned via the spelllike abliity editor in the editor. The same could be done for spells like call lightning, ecc…
In the end it depends on the power scale of this critter and what you intended it to do.