Onocentauros literally means "donkey-centaur": onos is Ancient Greek for "donkey". The oldest description seems to be from Aelian XVII.9:
Ὀνοκενταύραν καλοῦσι ζῷόν τι, καὶ ταύτην ὅστις εἶδεν, οὐκ ἂν ἠπίστησεν ὅτι καὶ Κενταύρων φῦλα ἦν…
There's an animal called the Onocentaur, and anyone who's seen one couldn't doubt that the race of Centaurs actually existed…
Different people interpreted Aelian's description in different ways. Given the name, some depictions (like the one you found) are just a centaur with a donkey's body instead of a horse's.
But notably, Aelian describes only two hands and two legs, while a centaur should have two hands and four legs. So we also see depictions like this:
(From Ashton's Curious Creatures, 1890.)
This seems rather, well…topheavy, to have actually worked in practice.
So some modern readers have looked at Aelian's description in a different light. Aelian doesn't claim to have seen an onocentaur himself, but he does mention that they can use their hands to run (on all fours) or to pluck fruit and leaves off of plants. Combined with the description of ἀνθρώπῳ τὸ πρόσωπον εἴκασται, περιέρχονται δὲ αὐτὸ βαθεῖαι τρίχες ("a face somewhat like a human's, surrounded by thick hair"), it sounds a lot like a misinterpretation of some sort of primate, maybe a chimpanzee.
"Does that mean that the Centaurs were also just horses that were seen incorrectly?" Maybe; some scholars have suggested that centaur stories originated with seeing people on horseback. But the details are lost to prehistory.
"What were the similarities between the onokentaros and the Centaur, other than appearance?" None, really, except that they were both said to be violent. I can't find any classical accounts of onocentaurs talking or being sentient, in particular.
"Were [onocentars and centaurs] ever seen to interact with each other?" Unlikely. As the quote at the top of the answer mentions, most accounts say that centaurs are long extinct (if they even existed at all—Aelian does express doubt on that point, despite his words above). They're fundamentally creatures of myth, while onocentaurs were actual animals that named sources had seen firsthand. The similar names are most likely because both centaurs and onocentaurs were both kind of like humans and kind of like other animals, not because they were seen as the same sort of being.