Especially when it comes to cloven-hoofed animals, many of these would have been animals that would have been, at least in general terms, familiar to us. Not necessarily familiar to us from Europe, though, since, in addition to pigs, bovines, and deer, there were also a number of antelopes. These were mostly members of the gazelle subfamily, although there were others, including some, for example, related to the modern sable antelope. The gazelles included Hispanodorcas, a small and slender antelope with slightly twisting horns, with fossils found in southern Spain. However, some were even closer to the gazelles of today, to the point that, if, like most people, you'd be pressed to tell the difference between a Dorcas gazelle and a Speke's gazelle today (or at least, to know which one was which), you'd probably not have identified these as anything different, either - although at least some of them were smaller than any living species, which might have helped.
The bovines were initially represented by the (relatively) small Parabos, a survivor from the previous epoch, but it was soon joined by at least three species of Alephis, known from early Pliocene deposits in both France and Italy. In life, they would have looked somewhere between a modern cow and a wildebeest, and, judging from their teeth, had a diet perhaps closer to the latter. Their horns flared outwards in a lyre-like shape, and they were also larger than Parabos.
The warm climate did not last forever, though, and around 3.2 million years ago, things took a decided turn for the worse. There had been a previous cold snap about 800,000 years earlier, but, geologically speaking, it hadn't lasted that long. Technically, this second harsher spell of cold (it may be when Greenland received its first glaciers), wasn't much longer itself, since it was reversed a mere 100,000 years later in the "Mid Pliocene Warming" event. But that turned out to be a last gasp, and by just 3.0 million years ago, Europe was firmly locked into a million-year slide of autumnal temperatures that would end, in due course, with the great Ice Ages of the Pleistocene.
This was good news for the voles, which had first appeared in Europe during the earlier, shorter cold snap, but less so for most other animals. Parabos, for example, died out, to be replaced by Leptobos, a larger, hardier, animal that looked much more like what we'd think of as a bovine. It was heavily built, with long horns that curved upwards and outwards from the back of the head, and it had strong teeth far better adapted to grazing on the tough grasses that the colder weather had brought. 140 cm (4' 7") at the shoulder, and at least 320 kg (700 lbs) in weight, and possibly a fair bit more, it would have been at least the size of the smaller domestic breeds of cattle today, such as the Jersey. It may have lived in a mixture of grassland and light woodland, and survived into the early Pleistocene.
Perhaps surprisingly, the weather didn't get cold enough to wipe out the gazelles, and, indeed, we see new species appearing at this time, such as the spiral-horned Gazellospira. Less surprising, perhaps, is the arrival of hardier animals, including some caprines that may be related to musk oxen. While animals more obviously musk ox-like did not really begin to prosper until the Ice Ages, these were at least larger and more heavily built than the caprines that had preceded them, and included Pliotragus and the "giant sheep" Megalovis. The latter is the larger of the two, and has been found from Spain across to Romania, with Asian representatives as far east as China. Whether it's genuinely a relative of modern musk oxen, or just a goat that happened to have a similar (though probably less extreme) habitat, is unclear, with arguments tending to focus on the shape of its relatively small, but widely spread, horns.
The colder climate also spurred evolutionary changes among European deer, which had not changed as dramatically in the earlier part of the Pliocene. For example, Croizetoceros had been around since the latter part of the previous epoch, developed new species that were not only larger than those that had gone before, but had more elaborate antlers, too. In absolute terms, it wasn't really all that large, being about the size of a modern fallow deer, but the shape of the antlers was quite different, with up to five prongs. It's also at this time that we see the first appearance of what may be direct ancestors of the modern red deer, and are certainly close relatives.
More significantly, the first of the "giant deer", the megacerines, appear in the late Pliocene. Representing an entirely new branch within the deer family tree, the early megacerines, while large, were not truly exceptional, although their antlers were quite spectacular. Arvernoceros is known from France and Spain, and, while it did not survive for long, geologically speaking, its descendants include the remarkable Irish elk of the later Pleistocene epoch.
Among the pigs, even the early Pliocene had seen the replacement of the older forms with animals such as Sus minor, a close relative, and possible direct ancestor of the modern wild boar, and hence also of the domesticated animal. The horses of Pliocene Europe, on the other hand, were certainly not ancestors of the modern forms. The previous epoch had seen the arrival of Hipparion, a three-toed horse, from Africa. Standing about 130 cm (4'4" or 13 hands) high at the shoulder, it would have been just short enough to qualify it as a "pony" under the usual modern definition.
While there were a number of species of Hipparion in Pliocene Europe, on the whole they were not living side by side, but in different times and places. H. fissurae, one of the better known species, for example, lived in Spain and Portugal during the earlier part of the epoch. Hipparion was adapted to grasslands, rather than dense forest, which, together with all of those gazelles, suggests that while southern Europe may have been greener then than it is now, it was clearly not dense woodland - the African savannah may be a better model, at least in some respects.
By the late Pliocene, the climate was cooler, but Hipparion continued to struggle on for the rest of the epoch. As the Ice Ages approached, it continued to adapt, with the last known species, H. rocinantis, having visible adaptations to a harsher climate and the spread of tougher grasses. But it was not enough, because a new kind of horse was approaching from the east: Equus stenonis. The first single-toed horse in Europe, this had come, by way of Pakistan, all the way from North America, where the success of the likes of Hipparion had been much more short-lived. Donkeys and wild asses may be particularly close relatives, although, at 140 cm (4'8" or 14 hands), it was a bit larger, and just on the upper side of the pony/horse boundary.
The arrival of this single-toed horse spelled the end for the three-toed kind, which disappeared rapidly from Europe. This change in the make-up of the fauna is a significant one, and is one marker for the end of the Pliocene, and the dawn of the Pleistocene. It is important enough that it has a name: the Mammoth-Horse Event. Mention of which, of course, brings us to the fact that there were also larger herbivores in Europe at the time...
[Painting by Heinrich Harder, copyright expired]