Technically, all of us are chimeras. In most cases it means nothing more than a few cells exchanged between the mother and her baby during pregnancy. Some of us may have also absorbed the fetus that was supposed to be our twin, and now this twin’s cells make up a part of our body. However, there are creatures that truly take the cake when it comes to chimerism. The biologists of the Classical era blamed the existence of those "monsters"--the beautiful abominations consisting of different animal parts fused together--on the magic lingering after Discord's banishment. I can't disagree with them. I have no evidence to contradict their hypothesis. To be fair, while I’m not a fan of such mythological explanations, I have to admit that the behaviour of these creatures is indeed quite peculiar and chaotic.
Chimeras
Creatures whose bodies seem to be a mixture of a tiger and a goat with a snake serving as their tail. They roam mainly the eastern parts of the Everfree where it slowly fades into the Badlands. You may know this place under the name Flame Geyser Swamp.
The main reason why they live there is the fact that they'd be easily outnumbered and pushed to the sidelines by timberwolf packs in the "normal" parts of the forest. However, you can't find timberwolves near flame geysers for obvious reasons. Before I tell you more about chimeras, I feel that some basic information about survival in the swamp is due, just in case you were in charge of emergency pie deliveries through it.
The two main dangers of the swamp are the fire and the bog itself. On a side note: if you want a hot mud bath, there are far better places to go. The bog can be avoided quite easily like in a normal marshland, and the fire eruptions are predictable. When you feel hard ground under your hooves, you are safe, but when it gets just a little muddy, you need to pay full attention to the bubbling and shaking underneath. When it intensifies, run. You are standing just above a flame geyser. And of course, watch out for the boggy areas when fleeing.
Chimeras have the advantage of being part snake and thus possessing the ability of sensing infrasound waves. When dragging the third head on the ground, they know exactly where all the geysers are. However, there are other problems they have to face.
Griffons have a similar saying to our "No bees, no honey, no work, no money." Their saying is, "Roasted pigeons won't fly into your mouth." Funnily enough, quite the opposite is true for chimeras. Food will come straight under their noses, sometimes even cooked. However, the problem is that the food is usually rather scarce.
A chimera doesn't need to eat everyday, but it can't starve for a month either--or, well, theoretically, one scaly third of it can. Each of the heads eats nearly the same food as the original animal would. The tiger part goes for meat, the snake after smaller creatures, and the goat after plants and sometimes baked stuff. For each head, there is an appropriate digestive system, meaning that not all three have to eat every time to get some sustenance for the whole body.
If a chimera is hungry for quite a while, the heads start to act more autonomously and return to their respective ancestral behaviour and weaknesses. The winner is usually the goat, at least for some time. When it's grazing happily, its sisters or brothers are starving. That often ends with the tiger's mind snapping and attacking the goat part, while the snake is waiting for the right moment to strangle the other two. This is the time when you might be able to escape a chimera unscathed as it’s going to be somewhat deranged and too busy committing its own murder. Or should it be called a suicide?
Due to the harsh conditions in the swamp, I still wasn't able to observe chimeras mating. Chimeras are one of the most sapient creatures around, but even I don't feel like seeking out one to ask such intimate questions. All I know is that a pair of chimeras then function as two individuals, not six.
Hydras
Chimeric creatures that bear an odd mixture of reptilian, amphibian, and possibly even cnidarian traits. They have both lungs and outer gills--visible as a bright red hem on their heads--and can switch between them. This grants them the ability to stay underwater for a long time, but also to survive on dry land.
Unlike chimeras, hydras are more fused together, and the individual animal parts are not really discernible anymore. Their heads also aren't as autonomous. However, each of them has control of the full body, which results in their lumbering gait and clumsy movements--in chimeras, the control is distributed between the heads. Younger hydras are more agile, simply because when they hatch, they have just one head. Their eggs look like those of a frog, and so do the legless "tadpoles" consisting of a head on a long neck that slowly tapers off into the tail. Hydras lay millions of eggs, but if one of them makes it into adulthood, it's a huge success. As they age, new heads start to bud around the "original" neck. You can recognize the oldest head by it being the slowest.
Even large adult hydras are excellent swimmers who are able to monitor everything that’s going on both underwater and above the water surface. So, whenever you are in the Froggy Bottom Bog and the surrounding open areas, be aware that you are being watched from everywhere.
Their keen senses make hydras rather skilled, non-picky hunters of smaller creatures. A duck, a small cragadile, or a frog, it’s all food. Luckily this saved the bog from frog overpopulation suppressing other marshland wildlife. Because somepony though that poor froggies needed more space and should be relocated there.
Despite their maw being able to gobble up a whole pony in a single bite, hydras won’t usually treat you as prey. If they attack you, it’s most likely going to be for intruding on their territory. However, you should be able to escape them by mere gallop. The true danger of hydras are the many parasites who feast on their thick skin. Many of these are large leeches or tick-like arthropods with long and sharp nippers, teeth and stingers whose host range includes ponies as well.
Manticores
A lion with a scorpion stinger and bat wings that are more or less useless. Younger manticores are able to glide with their wings, older can just propel themselves while running and flare their wings to look bigger and more intimidating. These are one of the laziest creatures I've ever met. Manticores spend almost all day napping, and when hunger finally calls, they search their surroundings for leftovers of a feast of another predator. Their search usually ends successfully on the hunting grounds of a younger cragadile.
When they do actually hunt, they prefer to use their paws over the stinger. Luckily, the laws of nature survived the amendment they went through with this creature. You might have heard of the rule "the bigger scorpion, the better" relating to the fact that the smallest of scorpions are the deadliest. Given their size, manticore's poison is even weaker than that of a wasp, though the stinger can still cause quite a lot of damage given its sheer size. However, you should be more afraid of their saliva, it's as thick as glue and has nearly the same quality. Also, when eating, manticores quietly hum to themselves. The reason for this is not yet clear. Albeit, do you have any reason for singing in the shower?
They roam the more central parts of the Everfree, and a lot of them are drawn to the edge of the rocky areas as there are always some fallen chamois to be found. They are strictly territorial, especially in the places they use for sleeping. It’s not a good idea to wake them up, trust me.
Quite many ponies ask this, so: Yes, females lack the mane, though the differences pretty much end there. Well, maybe they are a little more energetic. Before mating, the male incapacitates the female with the poison to calm her and then proceeds to the actual act. Up to two cubs can be born, and they stay with their mother for a few years.
So, how to deal with them? Don't interrupt their dreams, and you'll be fine. Otherwise, Tartarus help you, magic or strength won't.