Now, we see Geryon's double form of tail. This is another great example of Dante's presentation of Geryon, the Fraud. Dante shows fascination by all above descriptions, especially of the tail, both literally and abstractly no less than five times in this canto. They are usurers. Usury is not a sin against charity, but it is one against nature.
Money is meant to be consumed and to be spent.
Usury is a sin which gives away this unproductive substance and it is charged for its use as if these were two separated things. Meanwhile usurers were usually looking for false profits in the lives. They did not want to stay in one place at anytime. Therefore, the punishment is to be sitting under the rain of fire. The rain of fire burns life, but it does not nourish as nature rain is supposed to do.