I'm planning on writing a backstory-journal type thing for one of my characters. Before I do, I'd like to clear a few things up.

Exactly how aggressive are creatures supposed to be? More specifically the intelligent creatures, such as Minotaurs and Ogres. Would these creatures almost always be (for the most part) aggressive to the humanoid races, or could some be just passive or even friendly?

Would these more intelligent creatures be able to speak in any form of language? If so, would any speak Tamrielic?

Lastly, how common are Lycanthropic beings in Cyrodiil? (I know that isn't exactly a wildlife question.)

I've been reading books about creatures on TIL recently but haven't found much on these topics.

Well as you've discovered already, there isn't quite enough lore on those creatures to say much about them at all. Daggerfall however has some language skills that would allow you to talk to various creatures. So I reckon the more intelligent once do speak their own language.

Lycantropy is also hard to say, so I'd go with rather rare.

Most of this is my own stuff I've made up, though I've used tiny little things here and there to help me paint the picture.

Trolls seem to be demi-intelligent predators (much like their internet-based cousins) and seem to not care too much about who they attack if they're hungry enough. Otherwise, they tend to mostly be solitary creatures. Some are capable of language and even crude writing, though with no observed familial structure to learn from, how they learn or if they even have a consistent language is a mystery.

Ogres have some sort of loose society going on that seems to mostly consist of "people hate me, people hate you, let's hide from people together." They do commit the occasional banditry, carrying off with supplies, which implies a baseline understanding of how other societies function. They won't attack unless you get in their way, are hunting them and they believe they have no other option, or you've somehow (and easily) pissed them off. They do have a unique language and an observed religious behavior, especially towards Malacath. While mostly incapable of speaking Cyrodilic, most do seem to understand it.

Minotaurs seem to be the most intelligent of the three so far, with an observed clan structure, religion, magickal practices, language, and even rites of dead. They are, unfortunately, viciously xenophobic as well. They may hide from travelers or adventurers or if it's more convenient or if the traveler accidentally desecrates some holy shrine of theirs, they will often kill them in a very gruesome manner.

There are many conflicting reports about the Dreugh. Some say they are intelligent and of Man's equal. Some say they are little more than trolls of the water. Perhaps it depends on their life-phase, perhaps it depends on region. More research is required.

Will o' the Wisps are decidedly intelligent, capable of setting traps, luring people into them, and even evaluating a target's willingness to follow their lure. Though it is perhaps more due to the massive amount of magicka that they radiate. Additionally, young Will-O-Wisps seem to be less intelligent. Perhaps the more intelligent the life they feed off of, the more intelligence they also absorb. Or perhaps the ones capable of feeding off of Men and Elves are simply born with more intelligence. No one who has attempted to communicate with one has lived to tell the tale though, so who knows if they can speak or understand language.

While Dragonlings are rarely seen so far south, a clutch of them occasionally hatches. While it is possible to reason with them, at that stage in their lives, the most you'll be able to get out of them is why mutton is tastier than wolf. True Dragons tend to keep to the higher mountains, but Cyrodiil's central geography to Tamriel means that Dragon sightings there are a lot more common than other regions. This is likely why the Empire's symbol is a dragon.

There hasn't been a recorded instance of a were-creature in Cyrodiil since Uriel V's reign. That doesn't mean that they don't exist, nor does it mean that they don't wander in from other provinces. It just means that no one has reported one since then. Although difficult to prove, if there were werecreatures living in the civilized heart of the Empire, they'd likely follow the same proportions as elsewhere, with Wereboars and Werewolves being the most common, likely sticking to the Nibenay basin and the Colovian West, Werebears sticking to the cold mountains up north, and the more exotics living where the environment suits them best (Werecrocs in the swamps, weretigers in the gold coast, etc). Given that, I don't see how Lycanthropes can survive undetected inside the Imperial City. This author has, however, heard rumors that the Vampires native to Cyrodiil tend to not tolerate Lycanthropes staying in their territory too long, so perhaps there is another reason why we haven't seen one.

In addition to Mehrunes Dagon's post, goblins also seem to be highly intelligent with their own tribes, religious practices, and social heirarchy. Their society implies that they have their own language and, while warriors and shamans are hostile, goblin cooks and farmers are docile and timid.

Werecreatures...do they live in packs? From what we have seen so far in the ES, Werecreatures are solitary, but in the Bloodmoon, they were in packs. Most of them, the hound's at least. While the Player was a hound for Hircine, he worked alone. But in daggerfall, we seen some packs live in abandoned caves and dungeons, and it was not solely a strain, but both. Werewolves and Wereboars living together in a refuge. So, can it be possible a fellowship of Lycanthropes from various strains living together in some abandoned lair deep in the wilderness?

We can't be sure if this was typical behavior. Those lycans were under the direct command of Hircine, who may be able to command them to behave differently from their feral nature.

Lycanthropes! Lycan is used in the Underworld franchise, seperating the two clans from the movie. ;P

Anyways, I am aware Hircine was in command. But daggerfall had no Bloodmoon event on the time the game was taking place, and the Lycanthropes there, some of them, stayed iin packs. There was a mission where you, a vampire, had to go kill the Pack leader inside a cave. So from there, we can assume that they are indeed social characters like the Goblins and Trolls...but the majority remain alone.

I thought Willo-the wisps to be swamp gas

In that case, let's assume that lycnathropes operate on a loose pack mentality, with a dominant alpha male or female leading the pack. In the absence of an overmind like Hircine or a powerful sorcerer to command them, this is the basic hierarchy.

Let's assume that Goblins and Ogres are slightly more complicated and organized than the game mechanics suggest. Goblins are represented more fairly in the games than Ogres. I believe there's sufficient evidence to suggest there are tribal affiliations, hierarchies, leaders and probably a spoken and even written system of communication among ogres. They probably worship Malacath. Trolls also seem to belong to tribal alliances, probably led by powerful or old Trolls. Perhaps Udyrfryktes represent some evolutionary or cultural ideal for Trolls. Or perhaps they're simply mutants or outcasts.

Imps seem to have a very rudimentary culture. Their use of magic suggests a higher than average intellect for creatures. Minotaurs definitely seem organized and intelligent. The use of magic and weapons places them a cut above Ogres from what we've seen. There may be specimens of Ogre who are capable of these feats as well.

Centaurs obviously have their own language, and use a variety of weapons and armor, but are rare in Cyrodill these days.

Has anyone noticed we're talking about primarily Cyrodiilic creatures? Is it by coincidence that the creatures of the great civilized province are "civilized" as the humans and vampires there?

Yea, I noticed that. It's pretty strange...

From what I gleaned from Daggerfall, Centaurs had the most developed society of these and they don't associate with anything Imperial. Also, the OP implicitly asked for Cyrodilic wildlife.

He/she only specifically asked for Cyrodiilic lycanthropes, but asked pretty much about intelligent races, in general. What do we know of centaurs, though?

Plenty...in Daggerfall, one of the usable skills was Centaur language. You could communicate with and even do quests for them. That's a good indicator of a civilized society of sorts.