In the entrance of Caliban Prospero treats Caliban like a monster because he says “thou poisonous slave, got by the devil himself.” This metaphor shows that Prospero thinks Caliban is monster like a scorpion that has a poisonous sting and Caliban is too poisonous and is dangerous.
Caliban responds by saying “With raven’s feather from unwholesome fen drop on you both!” This metaphor shows that Caliban responds to Prospero by saying that Prospero and Maranda will drown in a poisonous swamp and that some heavy dew will drop on Prospero and Maranda. This says that Caliban and Prospero don’t like each other.
Prospero treats Caliban this way because firstly they don’t like each other and also Prospero believes that Caliban is a monster because he says that he was made by the devil and that he is poisonous which shows that he is devilish and will hurt anyone he touches.