While both franchises, take inspiration from the monsters of different mythologies and folklore, The Witcher and Shin Megami Tensei utilize them very differently. One great example of this is the Kelpie, who appears as a horse in The Witcher and as a monster in Shin Megami Tensei. Each version of the creature ties into its origins, albeit in different ways. However, one is much more accurate to Kelpie's Scottish origins, while the other takes more from similar tales from around the world.
Kelpies come from Scottish folklore, where they take on the appearance of a black horse that can transform into human forms. They are malevolent water spirits living near rivers or lakes which drag or lure people (especially children) to drown. As a horse, those who pet it or try to mount it get stuck to its coat and mane, which it uses to drag victims into the water.
Kelpie in The Witcher
While its unknown if the creatures themselves exist in the world of the Witcher (as they're mentioned but never encountered), Ciri's horse Kelpie ties into the real-world legend. Ciri first encounters this horse via a friend in the book, The Tower of Swallows. It responds to a magical bracelet and will come running when it's rubbed, even from afar. The imagery around it ties into the legend -- flowing mane, a pure black coat and scenes with water.
Later, the horse's owner dies, and Ciri takes the bracelet and horse. Upon discovering how superior it is in speed and strength to other horses, she names it Kelpie. While Kelpie doesn't appear in the games, it's mentioned during one of the main story quests when Geralt is searching for Ciri and must dive through water and deal with monsters to find a horse symbol and progress through elven ruins.
Throughout both the game and novels, Kelpie becomes somewhat of a bad omen, as death follows her due to circumstances with the Wild Hunt and other parties after her. Other characters in the story also begin to recognize her by her black mare. As a young maiden followed by death whose horse has Kelpie's description and magical qualities, it's clear that the Scottish legend has symbolic ties to Ciri's story. The Witcher takes the lore behind Kelpie and grounds it by turning it into an unfortunate circumstance associated with a girl and her horse, rather than a water spirit.
Kelpie in Shin Megami Tensei
Shin Megami Tensei's iteration of Kelpie is far more literal, as is the case with most of its demons. In earlier games, it appeared as a black horse, although Kelpie later became green, likely to associate it more with marshes and wetlands. More recent SMT and Persona games depict Kelpie as a half-horse, half-kelp-like monster. These design liberties help the creature fit into the game's dark worlds, and it's description comes from the real-world lore. Gameplay-wise, Kelpie uses Ice attacks, which fits with it being a water spirit.
Kelpie doesn't have much of a role in the SMT games outside being a staple demon for players to recruit who's been around since the beginning. In most games, Kelpie is simply another creature for the player to use. Shin Megami Tensei IV is the one exception, as it features a group of Kelpie fighting with another demon, and they decide to help the main character cross a river in exchange for taking out the problem demon. In this instance, it's really a water spirit, though it doesn't claim victims.
Which Version of Kelpie Is More Accurate?
Despite not being a literal spirit like in Shin Megami Tensei, The Witcher better captures Kelpie's legend as a black horse who causes demise, particularly from young women. A lot of descriptions surrounding the horse (whether its with Ciri or not) center around water, and it's often praised as a magnificent steed with a black coat. This fits the description, and with Ciri, a fair young woman, as its steed, The Witcher's Kelpie ties into the transformation in the legend.
While no one really drowns around Ciri or the horse, the two are often surrounded by combat and death through magic or other unfortunate means. Ciri herself often feels like a bad omen, and at one point, and after receiving her scar, she is described as looking like a demon having come into a tavern after riding up on Kelpie. Shin Megami Tensei's version is all surface level, simply being plucked from a legend and given a form, while The Witcher uses Kelpie in a way that ties in more of its lore.