Who is disguised as poor Tom? Why?
The Earl of Gloucester has been convinced by his villainous bastard son Edmund that his legitimate son Edgar is plotting to murder him in order to seize his lands and title. Gloucester has decreed Edgar an outlaw and authorized a manhunt. Edgar has disguised himself as a madman and calls himself Poor Tom. Lear, Kent, and the Fool, who are all homeless now, encounter Edgar hiding in a hovel in Act 3, Scene 4, where Lear and the Fool have a wild conversation with Poor Tom, who has to pretend to be as mad as both of them. Tom represents Shakespeare's view of the poorest wretches in England who live worse than animals. Edgar speaks some wonderful prose in this scene, including:
Poor Tom, that eats the swimming frog, the toad, the tadpole, the wall-newt and the water [water-newt], that in the fury of his heart, when the foul fiend rages, eats cow-dung for sallets, swallows the old rat and the ditch-dog, drinks the green mantle of the standing pool, who is whipped from tithing to tithing, and stock-punished, and imprisoned, who hath had three suits to his back, six shirts to his body, horse to ride and weapon to wear.
Edmund's villainy is fully revealed in Act 1, Scene 2 in the Earl of Gloucester's castle.