I read this when I was a kid, so some time in the 80's, may have come out earlier, though. I don't remember that much about it, other than that it was set in a post apocalyptic earth (I think United States, for some reason I've got California in my mind) where forest and wildlife has overgrown all civilization. There's a group of characters on an adventure. I think they are mostly mutated in various ways. One of them is a centaur-like creature. He has a horse body, and a human head and face and mouth, but he eats (mostly fruit) with a mouth that's in his chest. They keep finding "artifacts" that they find mystifying, but you as the reader can identify as various bits and pieces of technology, like a road sign, or a can opener.
Dark is the Sun by Philip Jose Farmer.
The centaur like being with a mouth in his chest is an Archkerri called Sloosh:
What was not familiar was the creature that had disturbed the beetles. The thing was huge, its head three feet higher than Deyv's. It had four massive legs ending in broad round pads. The body was shaped like a bean pod. That is, the main body was. From its front a trunk reared at a right angle. This was shaped like the torso of a man and had shoulders, two arms, a neck, and a head. The hands had a thumb and four fingers.
As you say, the Archkerri eats fruit through a mouth in it's chest, though it will eat meat as well:
Still, the time Sloosh took in finding and eating food continued to be a trial to Deyv. The Archkerri's mouth was concealed beneath the leaves on the "chest" of the upper body. Finding that out had been a shock to Deyv. It had seemed grotesque and also a little frightening. His grandmother had told him about a monster which was human-shaped, unlike Sloosh, but which had its mouth on its breast, and its diet was confined to human children. As a child, Deyv had been threatened with it when he didn't behave.
Sloosh would eat meat, including the rottenest carrion, when it was available. But mostly he ate fruit and vegetables, and he required great amounts of these. To speed up the search for food, the two humans would forage the edge of the jungle. They'd woven some large baskets from reeds, and they used them to store the fruit. Thus, they could walk faster, feeding Sloosh from the baskets. But collecting the food took much time, too.
This may be Hiero's Journey, by Sterling Lanier. I don't remember a centaur, but there is a mutant moose. Per Wikipedia:
The novel follows the adventures of a priest by the name of Per Hiero Desteen as he explores the mutant-infested wilderness of Canada and North America five millennia after an event called The Death destroyed civilization. Riding a mutant moose named Klootz, with which he is able to communicate telepathically, Hiero attempts to uncover what has become of some colonies that his abbey has attempted to establish. Hiero's eventual allies include Gorm, a telepathic black bear, and Luchare, a princess from the distant kingdom of D’alwah. On his journey he faces many dangers, including mutated humans, mutant beasts, and the evil forces of The Brotherhood of the Unclean
An excerpt from online:
The Computer Man, thought Hiero. That sounds crisp, efficient, and what’s more, important. Also, his negative side added, mainly meaningless as yet.
Under his calloused buttocks, the bull morse, whose name was Klootz, ambled slowly along the dirt track, trying to snatch a mouthful of browse from neighboring trees whenever possible. His protruding blubber lips were as good as a hand for this purpose.
Per Hiero Desteen, Secondary Priest-Exorcist, Primary Rover, and Senior Killman, abandoned his brooding and straightened in the high-cantled saddle. The morse also stopped his leaf-snatching and came alert, rack of forward-pointing, palmate antlers lifting. Although the wide-spread beams were in the velvet and soft now, the great black beast, larger than any long-extinct draft horse, was an even more murderous fighter with his sharp, splayed hooves.
Hiero listened intently and reined Klootz to a halt. A dim uproar was growing increasingly louder ahead, a swell of bawling and aaahing noises, and the ground began to tremble. Hiero knew the sound well and so did the morse. Although it was late August here in the far North, the buffer were already moving south in their autumn migration, as they had for uncounted thousands of years.
Morse and rider tried to peer through the road’s border of larch or alder. The deeper gloom of the big pines and scrub palmetto beyond prevented any sight going further, but the noise was getting steadily louder.
Hiero tried a mind probe on Klootz, to see if he was getting a fix on the herd’s position. The greatest danger lay in being trapped in front of a wide-ranging herd, with the concomitant inability to get away to either side. The buffer were not particularly mean, but they weren’t especially bright either, and they slowed down for almost nothing except fire.
The morse’s mind conveyed uneasiness. He felt that they were in the wrong place at the wrong time. Hiero decided not to delay any longer and turned south off the trail, allowing Klootz to pick a way, and hopefully letting them get off at an angle to the oncoming buffer.
The Taig, the vast boreal forest of conifers which had spread across the northern world at least a million years before The Death, still dominated the North. It was changed, however, with many species of warm country plants intermingled with the great pines. Some plant species had died, vanished entirely, as had some animals also, but most had survived, and adapted to the warmer climate. Winters were now fairly mild in the West of Kanda, with the temperature seldom ever getting below five degrees centigrade. The polar caps had shrunk and the earth was once again in another deep interglacial period. What had caused the change to be so drastic, man or nature, was a debated point in the Abbey classrooms. The Greenhouse Effect and its results were still preserved in the old records, but too much empiric data was lacking to be certain. Scientists, both Abbey and laymen, however, never stopped searching for more data on the lost ages in an effort to help shape the future. The terror of the ancient past was one thing which had never been lost, despite almost five thousand years. That The Death must never be allowed to come again was the basic reason for all scientific training. On this, except for outlaws and the Unclean, all men were agreed. As a good scientist and Abbey scholar, Hiero continually reflected on the problems of the past, even as now, while seeming to daydream in the saddle.
He made an effective picture as he slowly rode along, and not being without vanity, was aware of it. He was a stocky young man, clean-shaven but for a mustache, with the straight black hair, copper skin, and hooked nose of a good Metz. He was moderately proud of his pure descent, for he could tell off thirty generations of his family without a break. It had come as a profound shock in the Abbey school when the Father Abbot had gently pointed out that he and all other true Metz, including the abbot himself, were descended from the Metis, The French Canadian-Indian half-breeds of the remote past, a poverty-stricken minority whose remoteness and isolation from city life had helped save a disproportionate number of them from The Death. Once this had been made clear to him, Hiero and his classmates never again boasted of their birth. The egalitarian rule of the Abbeys, based solely on merit, became a new source of pride instead.
On Hiero’s back was strapped his great knife, a thing like a short, massive sword, with a straight, heavy back, a sharp point, a four-teen-inch rounded blade, and only one edge. It was very old, this object from before The Death, and a prize won by Hiero for scholastic excellence. On its blade were incised, in worn letters and numbers, “U.S.” and “1917” and “Plumb. Phila.,” with a picture of a thing like an onion with leaves attached. Hiero knew it was incredibly ancient and that it had once belonged to men of the United States, which had long ago been a great empire of the South. This was all he or perhaps anyone could know of the old Marine Corps bolo, made for a long-lost campaign in Central America, forgotten five millennia and more. But it was a good weapon and he loved its weight.
He also carried a short, heavy spear, a weapon with a hickory shaft and ten-inch, leaf-shaped steel blade. A crossbar of steel went through the base of the blade at right angles, creating what any ancient student of weaponry would have recognized at once as a boar spear. The cross guard was designed to prevent any animal (or human) from forcing its way up the spear shaft, even when impaled by the spear’s point. This was not an old weapon, but had been made by the Abbey armory for Hiero when he had completed his Man. Tests. At his saddlebow was holstered a third weapon, wooden stock forward. This was a thrower, a muzzle-loading, smooth-bore carbine, whose inch-and-a-half bore fired six-inch-long explosive rockets. The weapon was hideously expensive, the barrel being made of beryllium copper, and its small projectiles had to be hand-loaded by the small, private factory which produced them. It was a graduation present from his father and had cost twenty robes of prime marten fur. When his stock of projectiles was exhausted, the thrower was useless, but he carried fifty of them in his pack; few creatures alive could take a rocket shell and still keep coming. A six-inch, two-edged knife, bone-handled, hung in his belt scabbard.