Warding Gestures

The automatic display of arcane (and presumably protective) finger and hand positions in the wake of a surprising, stunning or physically traumatic experience. Most often this is the mano cornuta gesture—index finger and pinky raised, middle two fingers bent down to meet the thumb. While this gesture has roots in ancient Greece and is used in many societies worldwide, Americans may best know it as "throwing the horns", from its common use at heavy metal concerts (having been popularized in that context by Ronnie James Dio) or as the "hook 'em horns" from University of Texas, with the thumb and pinkie finger extended, to look like longhorns.

Not common, but it is found here and there spread across a few cultural media.

Examples of Warding Gestures include:

Anime and Manga

Many of the characters in Ranma ½ by Rumiko Takahashi display Warding Gestures after being hit with a large blunt object, or after suffering a Face Fault.
Another work by Rumiko Takahashi, titular character Inuyasha does this when Kagome says "Osuwari" (or "SIT, BOY!" in the dub), which sends him face-first into the ground.
Yet another work by Rumiko Takahashi, Maison Ikkoku, makes use of this on occasion. Though, since Maison Ikkoku places far less emphasis on slapstick comedy than Takahashi's other work, this trope is invoked far less often by comparison.
Several characters do this in Kashimashi: Girl Meets Girl, in similar circumstances.
In Rune Soldier Louie, the priestess Melissa occasionally makes Warding Gestures in response to shock or surprise.
Foxy from One Piece does this to shoot his Moron Moro-I mean, Noro Noro Beam.
Slight variant: Yuuta in Zombie Loan does this with his hands fairly often... and has them talk to each other, but then again, Yuuta's a bit nuts.

Comic Books

Doctor Strange commonly uses mano cornuta during spellcasting.

Literature

From the Wheel of Time, one of the Seanchan Empire's cultural differences between the Westland are their use of hand gestures in this fashion, including the horn.
"The Holy Horns" in Small Gods, used as a holy sign for the church of Om, since one of Om's physical manifestations in the past was a large bull.
In The Dark Tower, Roland and other characters occasionally use the mano cornuta when evil powers are mentioned aloud, which is pretty close to its historical use in Real Life.
Miles Vorkosigan has put up with people making hand gestures at him "to ward off mutation" for his entire life.
Many of the peasants that Jonathan Harker, and later Mina and Van Helsing, encounter in Dracula do this using the typical "throwing the horns" gesture.
This is used by worshippers of Ulric in the Warhammer Fantasy Battle world as the sign of the wolf. At least in the Gotrek and Felix novels.
In Tortall, people who fear mages often do the 'sign against evil' when they see magic being done, or hear it mentioned. It's stated in the back of one book that it's a five pointed star, and the books specify it's made with one's hand. Luckily for them, no mages seem to take offense to this.
In Percy Jackson and The Olympians, their Ancient Greek warding-off-evil gesture is a three-fingered claw over the heart, then pushing the hand outward.

Live Action TV

Guinan in Star Trek: The Next Generation does a version of this when confronted by Q and doesn't lower her hands until he lowers his first. Given what we'd seen of Q's power (even with this only being his third appearance) it's impressive that Guinan even thinks she can do something about Q if he decides to attack her. Q's reaction (involving his own gesturing in her direction) indicates that he takes Guinan seriously. Just who is this bartender? The Expanded Universe books reveal that the whole Nexus thing means a bit of her is still there - even before, due to the way the Nexus works. This makes her more than your average bartender, though far from Q's power level.
For some reason this is part of the basic stance of Minbari martial arts.
The Senshi in Pretty Guardian Sailor Moon often make the fox-head gesture before or during attacks.

Music

Used universally in the heavy metal subculture, and increasingly in other rock & roll settings, as a general sign of appreciation. The original reasoning for Dio's usage of the gesture is that when he joined Black Sabbath, he wanted to use a hand sign on stage as Ozzy was known for using the peace sign, but he didn't want to use the same symbol as Ozzy, so he opted for the symbol that his Italian grandmother had used to ward off the Evil Eye. It also spawned the smiley "\m/" before there was "🤘".
- Given the tradition of Heavy Meta, it should not be surprising that there's at least one song dedicated to this gesture. "Sea Of Horns" by Absolute Power (and then they posted a photo of the fans doing just this, and got a giant hand in this gesture raising from the sea on their cover, and in The Merch, too).

Raise your hands…
Salute…

Tabletop Games

The "Aquila" hand gesture in Warhammer 40,000, in this case performed by linking your thumbs together fingers straight, across your chest palms towards your chest. It's that butterfly people make with their hands when playing with shadows (and looks like the iconic double-headed eagle of the franchise).

Theatre

Serafina makes the mano cornuta gesture for warding purposes several times in Tennessee Williams' play The Rose Tattoo. She also talks about horns as a symbol of cuckoldry; just to overdo the symbolism, there is also a goat in the cast.

Video Games

Faust in the Guilty Gear series makes Warding Gestures after getting hit particularly hard.
The Undead building construction icon in Warcraft III shows a skeleton hand throwing the horns.