Ongon were made to be replaced. A figure serves a particular relationship with a particular spirit, and when the reason for it passes, or when the household changes, or when the figure has taken as much feeding as it can hold, it is disposed of and another is made. Nothing about the object argues for keeping it. It is cheap to produce, made from materials any herding household has to hand, and there is no tradition of an heirloom ongon.
Two waves of destruction sat on top of that impermanence. Buddhist authorities in Mongolia moved against ongon at several points, gathering and burning them as the visible marks of a practice competing with the monasteries. The twentieth century was far more thorough: state atheism destroyed monastic and non-monastic material alike, and the making of ongon stopped in the open.
The consequence for anyone studying them is uncomfortable. The historical examples that survive are, overwhelmingly, ones that left Mongolia in the hands of outsiders, collected by travellers, missionaries and ethnographers in the nineteenth and early twentieth centuries and now held in European and Russian museums. The ones that were kept properly are gone, because keeping them properly meant not keeping them. Figures have been made openly again since the 1990s, and those are new.