There are plenty of wondrous animals to steal ideas from, in various legends. On the boar theme, how about the Aetolian boar from Greek myth? (Memorably reinvoked in Zelazny's
This Immortal as the Black Boar of Thessaly.)
A dreadful boar.—His burning, bloodshot eyes
seemed coals of living fire, and his rough neck
was knotted with stiff muscles, and thick-set
with bristles like sharp spikes. A seething froth
dripped on his shoulders, and his tusks
were like the spoils of Ind [India]. Discordant roars
reverberated from his hideous jaws;
and lightning—belched forth from his horrid throat—
scorched the green fields.
Eyes of flame, spikes protecting his neck, tusks the size of an elephant's, and lighting breath. And so ferocious, it drove the people of the countryside to seek shelter behind the walls of the city, as if besieged.
Though the most interesting parts of the story aren't the boar itself, but the intrigue surrounding it. The boar didn't just happen, it was sent to plague a region. By a god, who was pissed off because the king forgot to honor her in an annual rite. In response, the king sent out messengers to seek out those who would try their hand at slaying the beast, and earning its hide and tusks as trophies. One of the heroes is a savage huntress representing the scorned god, another is the son of the king, and there were assorted other kings and princes, and others that were just heroes.
A long list of petty, mean, and stupid things happened during the hunt. Before it even ensued, most of the heroes, driven by jealousy and prejudice, refused to hunt with the huntress. Though ultimately, they were won over by the besotted prince. During and after the hunt, many heroes turned on each other, trying to keep the spoils to themselves. Accidents happened, like the hero who inadvertently killed yet another king, who happened to be his father in law, and thus had to flee.
Ultimately, the huntress took first blood, and the prince slew the boar. But out of love and honor, gave the spoils to the huntress. Which precipitated another fight, because of the sons of another king claimed that if the prince refused to accept the spoils, it belonged to them by right of birth. In a fit of pique, the prince slew the sons of the other king. But his mother was sister to the other king's sons, so she magically killed her own son in return. All of which was due to the god's meddling, in order to get revenge against the king.
Even after it died, the boar's hide was taken to a temple where it rotted and shed hair and bristles in majestic glory. The tusks were taken by a great empire, and one was broken.
To turn that into a game scenario, start with criers, rumors, or posts on billboards that notify the PCs of a great event. Bring in all kinds of heroes and adversaries the PCs have met or heard rumor of, including NPCs of high stature. Have a celebration before the hunt, where the PCs can ask questions, observe, spy, and otherwise figure out who else is there, and open and secret motivations, including conflicts. They might spot liaisons or suspicious gatherings, overhear plans of ambush, see spells being cast, and so on. Then have at least one incident happen that night, to demonstrate, even to the less perceptive PCs, that hidden trouble boils beneath the surface of the gala.
In the morning, have the hunt. The hunters split into various bands, which head off in different directions, based on different guesses about the location of the beast (drop hints enroute or the night before). Some bands hunt. Others try to hinder other parties, setting traps or trying to lead them astray, with magic or mundane skills. Others shadow other bands, hoping to steal the kill. Some even ambush. Mix in the cultists of the offended god, who are there at their god's behest, not to directly attack, but to use magic and mundane wiles to stir up trouble and aggravate tensions.
The PCs might be attacked, run across an encounter, run across the aftermath of an encounter, stumble on a tragedy, and may even be asked to bear witness or make a judgment. And, if they choose correctly, they might run across the beast. The fight of course attracts the other hunters, bringing in the PCs as needed, and ultimately leading to a multi-sided conflict. The different factions make attacks of opportunity, usually at least plausibly deniable, at their rivals. This might escalate into outright attacks, if sufficiently provoked or magically infuriated by the cultists. In general, make it a mess of a mass melee. Treat the boar as a boss monster, with enough hp, defenses, and the ability to attack groups or multiple opponents to make it a formidable foe even in the face of Lancester's law. Maybe even have a horde of thunder piglets, or the land turning against the hunters, or lighting from the sky, or sylphs or other spirits coming to its defense. It might even be spontaneously resurrected several times, in a blast of lightning.
Then even after the boar is killed, they still contest the spoils, making up arguments and excuses that are variously valid. The PCs can take sides, stay neutral, or try to make peace. But most resolutions will involve the spoils being separated, and where they will be taken depends on the NPCs (or the PCs) who gain the goods. First blood and the killing strike should be the strongest arguments.
However they're distributed, the ideal is to make the trophies them long-standing fixtures of the campaign. Have them staged in important locations, like in the hall of a mighty lord, on the altar of a great temple, above the gates of a grand guild, or so on. Give them magical powers, but make them more domain-level than personal -- the PCs can't wield them as weapons or armor, for instance. But they might dispel the clouds in the hex around them or bring rain in drought; or maybe they can be rubbed to endow magical properties on items; or perhaps they ward a great area against certain creatures; and so on. Wherever they go, have them affect the course of the campaign.
Still rough; the factions are still vague ideas rather than concrete. But hopefully, you get the idea.