In The Merchant of Venice, Antonio is unable to pay his bond to Shylock and Shylock is therefore claiming his "equal pound of your fair flesh" (I.iii.144) from Antonio which is the forfeit to which Antonio agreed so that Bassanio could woo Portia. Antonio never expected to be in such a position but is prepared to accept his fate. The Duke is trying to persuade Shylock to show mercy but Shylock is adamant. He refuses to explain his feelings except to reiterate that he has a "certain loathing" (IV.i.60) for Antonio.
Gratiano who is Bassanio's friend and who will marry Nerissa, is pleading for Antonio's life. He says that Shylock's insistence that Antonio must deliver a pound of flesh (and therefore bleed to death) is making Shylock seem like an "inexecrable (cursed) dog" (128) to the point that he almost believes that Shylock is the reincarnation of an animal because he has no compassion. Pythagoras, an ancient great Greek mathematician and philosopher believed that, upon death, the soul of another- and even animals and pets- could "transmigrate." In this instance, he is suggesting that Shylock was once a wolf and now inhabits the body of a man. Shylock's wishes are so bloodthirsty that Gratiano is inclined to agree with Pythagoras's viewpoint, and therefore doubt his own faith, and believe that Shylock is the reincarnation of a vicious animal.