Their song is hypnotic, and all who hear it are immobilized. Legends tell of shipwrecks that were caused when the sailors were captivated by a Syrentide's song.
Syrentide is a bipedal marine Pokémon that resembles a siren. It has two arms ending in hand-like fins with three digits, and a tail ending in two fins, with a golden trident-head attached to it. It also has a wave-like hairstyle, a golden trident-head hairpiece, two shiny black eyes, a golden collar and two short fins for feet, with an additional fin on its back and on its tail. It is primarily dark-blue in color, and its whole body is patterned like the waves of an ocean, with seafoam patterning its wrists, hair and stomach, giving it a regal appearance. Its face is white and has four protrusions, giving it the appearance of some sort of mask. Syrentide's hypnotic song can immobilise people. Legends tell of Syrentide captivating sailors with its voice and leading them to their doom. Syrentide is a female-only species with no male counterpart.
As Mega Syrentide, it becomes even more regal-looking and impressive. Pink-tipped wing-like fins protrude from its face, and its hairstyle grows longer. It also gains two additional fins on its shoulders, which are white with light pink ends like its facial fins. The trident on its forehead now has five prongs, and is more akin to a tiara, with a small blue jewel in the center. Its golden collar becomes larger and fancier, and is adorned with a blue jewel and a blue pattern, giving it the appearance of wearing a pendant around its neck. Most of its fins become a shade of dark purple, with its hands becoming larger and possessing a lighter-colored purple cuff and a blue jewel each, and its leg fins becoming frilly and tipped with the same shade of lighter purple. Its tail grows longer, and it gains two long pink fins running up the sides of the tail. The tail-fin at the end is larger and fancier, pink-colored, and has a larger trident attached to it, with the middle prong ending in a point and the side prongs ending in spirals.
Syrentide is based on a siren, a creature from Greek mythology that would lure sailors to their doom with an enchanting song. It also shares characteristics with mermaids (which are sometimes depicted as similar to sirens in popular culture) and its trident-like tail may reference Poseidon, the Greek god of the sea.
In addition to making the sprites more distinctive, Selkid and Syrentide's altered Shiny sprite color scheme may have been intended as a reference to another Nintendo property in the form of the Zora Champion character Mipha from The Legend of Zelda Breath of The Wild. In addition to the color scheme, the character was also female, and was a Zora, a race in the Zelda series with aquatic attributes. Breath of the Wild also came out about a year after the distribution of Pokémon Uranium online, and the patch featuring the changed sprite was made available about 3 months after the game's release.
Syrentide's name appears to be a combination of a misspelling of siren and tide.