A known idiom in the supernatural universe of Teen Wolf is that a person, when they are turning into a werecreature or shapeshifter, they will not necessarily transform into the same species as intended. For instance, they may not transform into the same shape as their creator. The creature they will transform into will "reflect the person they are." Derek Hale was the first person to speak this idiom in the series.
The saying proves true in relation to a number of werecreatures seen in the series.
In Season 1 Peter, with his new status as an alpha and his knowledge and experience as a werewolf, was able to shapeshift into a grotesque, hulking, beast-like form that could stand upright, and run on all fours. This monstrous form was most likely the result of his thirst for revenge. Peter lost this shapeshifting ability both because of Derek killing him usurping his alpha status and coming back from the dead leaving him greatly weakened. As the fanatical, power-crazy Demon Wolf, Deucalion could effortlessly transform into a human shape with dark blue-gray skin with more demonic than wolf-like features. Scott saved young Lori Talbot from being executed by an assassin and then attempted to kill the man out of injustice. Giving into his bloodlust, Scott began to transform into a more monstrous and bestial shape with black-gray skin and a darker alpha eye color. When he regained control and spared the man's life, Scott reverted back to his normal form. When Sebastien became a werewolf, his psychopathy, his evil, twisted personality affected his werewolf transformation far beyond that of any shapeshifter ever seen. His werewolf form was that of an overlarge quadrupedal beast form. His body was oil black, thickly muscled, outfitted with red veins and and an overlarge lipless mouth Sebastien could transform into a beast after his body became shrouded in dark energy and shadows that he could summon, forming his werewolf body.Peter Hale
Deucalion
Scott McCall
Sebastien Valet

Jackson was an extremely narcissistic young man who had deeply rooted insecurities over the fact he was adopted and never knew his birth parents. He had an unhealthy drive to make people proud of him; his personality lacked an identity. Jackson's shape was eventually revealed to be a reptilian shapeshifter called the kanima. A kanima has no personal identity or motive, only that of its master.