Elder Guardians were introduced in Minecraft's recent Bountiful Update. They are boss monsters found within Ocean Monuments: they attack you with their laser, they put a slowing debuff on you, and when you hit them, they prick you with thorns. And they also make phenomenal pet fish, apparently.
One of the clever players in question, CezoramLHS, decided to take an Elder Guardian in upon finding one in an Ocean Monument's top room, close to sea level. And so, Cezoram opened up the dungeon, laid down some minecart tracks, and carefully (with the use of invisibility potions) pushed the angry fish to its newly-built aquarium.
And, it looks like, he wasn't the first one to do this. A few days earlier, a player called patchyweasel also caught an Elder Guardian in a jar using similar methods, noting how difficult it was to pull off both because of having to build underwater, and having to work under the influence of the Guardian's digging-and-mining-speed-slowing debuff.
Beats having a pet wolf, in any case.
You can read both players' stories in the embedded albums below.