Tren Krom from BIONICLE exists as a Shout Out to Lovecraft. However, he actually isn't an alien or other; he was created by the Great Beings just like everything else in the Matoran world. Despite this, he's got the looks — tentacles and all — and causes insanity in those who look upon him. He was also one of the first things to exist in the Matoran world, managing it before Mata Nui, who contains the entire Matoran world in his body, was activated.
Tren Krom: You think me an alien... an "other"... But I am of the substance of this universe, and I walked here long before you or even Mata Nui himself. — Federation of Fear
While we're at it, Makuta was also described like this as first- a kicking, screaming, lashing, thing of pure malevolence.
And now there is Annona, which has been described as a small, star-like thing with tentacles and a room chilling aura. It eats dreams.
The first Rahi were tentacled monstrosities and other madness-inducing sea beasts. Akin tho Tren Krom, they are actually naturally occurring in the Matoran world, but they are still unspeakably ancient and strange.
