What are the demons at the end of Madoka Magica? What are the demon-like entities shown at end of Madoka Magica, which Homura is now fighting instead of witches?

And what does Homura's explanation mean, regarding how these demons replaced witches?
During the conversation between Kyubey and Homura near the end of the final episode of the series, Homura mentions that although Madoka's wish prevented witches from being born, the curses from the human world still remained and continue to haunt humanity in the form of "magical beasts" (魔獣), which are demonic forces of unknown nature. 

The new role of magical girls now is to defeat these demons, gather their curses in the forms of small cube fragments, and have Kyubey harvest them, in order to slow the effects of entropy on the universe.