Adorable Abomination

Also, Vita is delightful. Such an adorable walking violation of the laws of god and man.

—Burnnote commenting on a chapter of Vigor Mortis

Let's face it. We all love Cute things. Some like Cute for Nosta lgia. Some like cute for... Perverted reasons. There are even people who think ugly things are cute! But sometimes Cute overlaps with Terror. Bonus Points if it's something from an animated work. Can overlap with Killer Rabbit.

Compare Grotesque Cute. Not to be confused with Cute Is Evil, but it can overlap.

Examples of Adorable Abomination include:

Anime and Manga

Higurashi. Practically everyone in it is cute, but sooner or later they all end up killing one another.
Elfen Lied. There was a scene where a little girl was in a house, and two firefighters came to get her, but she kills them both on the spot.
Kyuubey from Madoka Magica. Given that he's actually a Starfish Alien with Blue and Orange Morality, and that at one point he eats his own corpse, yeah, he qualifies.
Kamineko from Azumanga Daioh. Sure, he looks cute, but he has a grin that resembles a bear trap and a bite to match. At Times they'll even play what sounds like the theme from Jaws when he appears!
Not even Yu-Gi-Oh! is safe. Behold (Cue Drum Roll)... Kureboh!
Hazuki from Moon Phase. A cute little girl right? But when the moon comes out and she goes into Luna mode... bad things happen.
Kirby, at least according to Kirby: Right Back at Ya!. Also, Gooey from the games, who is a piece of Dark Matter who broke off, turned good, and became Kirby's friend.

Film

The Stitchpunks from 9.
Besides Plushes, some of the WALL-E items are kinda creepy.
Toothless from the How to Train Your Dragon movie can switch from Cute to Fierce by changing the dilation of his pupils.

Live-Action TV

The Cybermat from Doctor Who. It's a little silver thing that scoots around the floor. And then you see the razor-sharp teeth.

Tabletop Games

Pokethulhu and Mortasheen display two very different ways to apply this to the Mons genre. The former is full of cute, adorable creatures that love to eat souls, while the latter is full of frightening, hideous monsters that become friendly and loyal when tamed.

Video Games

Some species of Heartless and Unversed from Kingdom Hearts fall into this trope. All are Eldritch Abominations; some happen to be adorable.

Web Comics

Erma, in Erma, is a Stringy-Haired Ghost Girl grade-school student.

Web Original

Whateley Universe example: Josie Gilman's cute little black kittycat. It loves to be hugged, it loves to be fed, it... oh my god it's an Eldritch Abomination with a huge gaping maw!!!
- The cuddly stuffed cabbit Generator made for Tennyo should count too. Especially when the claws and fangs pop out.
SCP-999 from the SCP Foundation is arguably the friendliest SCP, it's a cute, happy, ball of yellow goo who loves giving people hugs. Only the 05 Council and their most trusted underlings know that SCP-999 is, in fact, the offspring of the Scarlet King, making it a baby Eldritch Abomination, and possibly the strongest weapon the Foundation would have against its father's apocalyptic goals.
- SCP-053 is believed to be another offspring of the Scarlet King. While far more dangerous than SCP-999, she has the appearance, personality (and unfortunately, naivety) of a small child.

Western Animation

This is what the people in charge of Heloise's personallity in Jimmy Two-Shoes seem to want to protrayed as. It doesn't work