You’ve probably heard of Afrotheria. A line of placental mammals expanded into all kinds of niches, back when Africa was an island continent. Elephants, hyraxes, manatees, tenrecs, aardvarks, elephant shrews, and golden moles are the existing members of Afrotheria. There used to be more: things that looked like a rhino (Arsenoitherium), for example. The thing is, they had diverged very far. Biologists had realized that there was a relationship between elephants, hyraxes, and manatees, but they didn’t know that aardvarks, tenrecs and golden moles were in that same clade. Without genetic analysis, it’s not easy to see that an apparent mole is really more closely relayed to an elephant. They’ve simply gotten too far into the part.
South America was also an island continent with its own placental mammalian lineages, and some of them filled many different niches. One, Xenarthra, has survivors: anteaters, tree sloths, and armadillos. The other, Meridiungulata, had members that looked like horses, camels, rhinos, hippos, rabbits, even chalicotheres. Most lost out after the formation of the isthmus of Panama let in North American competitors, and Amerindians finished off the the last survivors ( like Maucrauchenia and Toxodons) By the way, you will see South American paleontologists defending the competence of their extinct fauna, making excuses for their defeat by invaders from the North: it doesn’t get much stupider than that.
I’m wondering of any of the Meridiungulata lineages did survive, unnoticed because they’re passing for insectivores or rats or whatever, just as tenrecs and golden moles did. . Obviously the big ones are extinct, probably the others as well, but until we’ve looked at the DNA of every little mammal in South America, the possibility exists.