-giant animated chessmen/woman? made of marble or something even harder to kill, oh armok dont make them steel. yield masterpiece statue after death. semi-mega beasts.
-great race of yith, their form on Earth. semi-mega beast.
-Chinese long(or lung, depends on schools of romanisation). basically a superior type of being shaped like a giant serpent, with claws, often associating with water bodies, also can roam in midair. probably semi-mega beast.
-sphinx?
-typhon. a hundred of heads. many arteries to cut.
-cerberus. a family reunion!
-a hill-sized slime?
-lotr ents. i mean, they are gigantic and extremely long-lived, immune to most types of conventional attacks(dealt by creature of dwarven size) and sentient.
-huge tortoise which bears a small island on its back.
-firebird. it's a popular mythological creature through out many cultures, afaik. though the revival of phoenix could be hard to implement.
-big bee? size of a soccer ball but...necrosis sting! and it doesnt not die after stinging something. well just as a regular dangerous beast maybe.
-warrior from valhalla. maybe even not a beast at all. despite how and why it get out of there, it had been a great warrior when it was alive, and it'd been sparring everyday after its death in valhalla. since it's dead but not undead, it possess, still, flesh and bones, though it suffers no pain, fear nor fatigue. it also mastered out every single possible combat skill, and quite superhuman attributes.
basically i think, you can upscale any ordinary creature to a mythological/fantastical dimension and give it one or two strong points... there, a new (semi)mega beast.