Extreme trans-Neptunian Objects (ETNOs) are some of the farthest objects observed so far in the solar system. This family is characterized by large semi-major axes (a > ~250 au), large eccentricities (e > ~0.5), and large inclinations (I > ~15º). At such large distances from the Sun, the perturbations from the known giant planets cannot easily account for their existence. Several hypotheses have been proposed to explain their origin, among which the referred to so-called Planet 9 occupies a preeminent place. However, the recent discovery of a low-inclination ETNO may provide new insights into the origin of this family. In this talk I will briefly review the orbital characteristics of ETNOs, some of the proposed hypotheses to explain their origin, and our contribution to the matter from the recent discovery of 2016 SD_106, a peculiar object, both physically and dynamically, currently located in an extreme orbit with low inclination.