Based in part on some of the above
Creatures: Ent (generic tree warriors of the forests, found in good forests; prone to rage to encourage violence to outsiders) Revenant Warrior (animated undead suit of rusted steel armor haunted by a spirit of a long-dead warrior; on death, leaves behind a masterwork steel weapon) Quetzal (giant feathered serpent, savage tropical lands, capable of sucking blood) Abomination (various conjoined monsters resembling humanoid torsos in various configurations, found in caves and evil lands.)
Semi-Megabeast: Thunderbird (smaller relative of Rocs, able to disorient an enemy with a deliberate flap of its wings to create a horrendously loud booming noise,) Dreamstalker (Mr. Sandman described above, though for balance, limit discharge to three times a season, to necessitate either numbers or distractions to fight it; sleep syndrome lasts a week.) Mountain King (yeti-like creature that dwells in mountains, can inflict a resistible freezing touch once per week that can potentially be fatal.) Wyvern (two-legged winged dragon, think Skyrim's dragons but able to breath skin-dissolving acid or fire) Bloodtusk Mammoth (inspired by Far Cry primal, a massive fuzzy elephant with four crimson tusks found in cold regions) Chimera (the smaller version of the Great Chimera, minus poison)
Megabeast: Anantesesha (gigantic serpent based upon SCP 3000, secretes and spits a gray fluid which makes affected enemies dizzy, nauseous, and tanks thier agility, to represent it using this fluid to mess with a victim's mind,) Elder Ent (A gargantuan Ent with hard-to-pierce bark skin, and sap that if exposed to, causes creatures to suffer from necrosis at the point of contact.) Grand Quetzal (A massive version of the common Quetzal capable of flight and speech and thus taking over civs; can breath fire as well as suck blood out of enemies that survive an initial bite; flame attack is limited to once per week to encourage it to make use of that damned bulk.) Behemoth (A massive bearlike beast with horns and reptile skin.) Leviathan (as a stand in for the Kraken, depiction in the second image Hugo offered up) Great Chimera (a gigantic griffon, with poisoned talons)