It is a custom for Imperial Family, who are demoted from nobility to subject, to establish a new prince's house, but there was another option of being adopted by the family who are common subjects.
皇族は臣籍降下に際して、新たに一家を創設するのが通例であるが、臣下の養子（猶子）となる形で臣籍に降下する例もあった。