Set during the Avengers.
Being the God of Fire, Loki had always been somewhat hydrophobic. Being a jotun, he realized that his fears were completely unfounded, and water would not drown him if he had the capability to turn into a fish.
Loki stared at the ocean underneath the SHIELD helicarrier, marveling at this realization. His brother was next to him, moving closer to lock the muzzle over his mouth.
He pushed at Thor lightly with cuffed hands, "Wait." Stupidly, the other stopped.
Loki crouched precariously over the edge, close enough that he would fall if he leaned in any further. The other Avengers and the one-eyed man were making their way over, noticing that Thor was in no hurry to take his wayward brother to Asgard.
The trickster paid them no mind, his eyes locked on the glimmer of scales barely visible through the blue.
Then he took a deep breath and yelled, "Jormungand!"
SHIELD agents and Avengers alike yelled curses and took several steps backward when a monstrous reptilian head rose from the sea, stretching out effortlessly to meet the airborne helicarrier. Water rolled off its scales- -each half the size of a full-grown man- -splashing those onboard with a generous amount of salt water. Its eyes, slitted and yellow- -and as big as trucks- -blinked sluggishly, focusing on the small figure raising an entreating hand up to it.
The operatives began their attack, only to have all their best weapons bounce off the scales uselessly. All except Thor, who just stood motionlessly, gaping at the snake.
Loki was drenched to the bone, and he shuddered in disgust. He truly hated the water.
The serpent opened its mouth and let out a single, hissed word.
Motherr..sss...
"What the fuck did it just say?" Clint exclaimed, firing another arrow at its thick hide
"There is a snake head the size of Alaska staring at us, and you're worried that it can talk?" Steve said.
"Alright, fess up, who gave birth to Snakezilla here?" Tony yelled, slamming ineffectual metal fists onto the scaly hide.
"I may have birthed you, but I am still your father, young man!"
All eyes turned to Loki in disbelief. Loki, being Loki, ignored them.
What isss it you want?
"I just wanted to say hello, and apologize for the hundreds of birthdays I have missed." The serpent's parent said blithely. "Now that you have sufficiently scarred the mortals, you may leave."
That'sss it? At his parent's nod, the snake appeared to survey the scene, noting the handcuffs on the trickster's wrists. It snorted. Give my regardsss to my ssisster. The way you're going, it won't be long before you ssseee herr.
"That is uncalled for." Seeming to snap out of his stupor, Thor waved at his nephew happily. Loki glared at him and turned back to his son. "And Jor? Don't forget to eat your uncle Thor at Ragnarok."
I won't.
With that, the snake disappeared under the rolling waves.
There was a long pause.
"Why did I not just summon Jor instead of the Chitauri to conquer Midgard?" Loki asked no one in particular.
The Avengers regained their bearings and hastily muzzled the God of Chaos, shoving the Tesseract into Thor's hands and all but demanding that he take Loki away before he got any more ideas.
RnR!
According to Norse Mythology, the giantess Angrboda was Jormungandr, Hela, and Fenrir's mother, but Loki was the one who gave birth to them. There is also a prophecy called Ragnarok about the apocalypse, saying that Jormungandr and Thor will kill each other, while Fenrir kills Odin.
In the myths, Loki's children are banished, and he can't visit any of them except Jormungand, who was cast into the ocean. But since his element was fire, he couldn't stand going near large bodies of water.