This might be the 23rd episode of the animated series, Tara Duncan, titled "The Gold and the She-Dragon" (2010).
The Wikipedia description for the series as a whole:
Tara Duncan (also known as Tara Duncan: The Evil Empress in the United States, which is the title of the eighth novel) is a French animated series produced by Moonscoop, very loosely based on the novel series. The show premiered on 27 June 2010 in France on Disney XD, and aired through 19 June 2011 but was cancelled for reasons unknown, much to the disappointment of fans.
The Wikipedia description for this specific episode:
Master Chem is in love with an adorable dragon named Mel, but when he invites her to spend the weekend with him on Earth, she becomes the number one suspect for a robbery in which gold was stolen from one of Rosemond’s banks. Tara and her team take it upon themselves to prove Master Chem’s innocence to stop him from being relieved of his duties, and to allow him to enjoy a carefree romantic weekend.
According to the Wikipedia page, there are four main characters on the show: two girls (Tara Duncan and Princess Gloria Daaval), a boy (Caliban "Cal" Dal Salan), and Tara's grandmother (Isabella Duncan).
The description of Princess Gloria Daaval reads as follows:
Princess Gloria Daaval (English dub: Kelly Metzger) is Tara's best friend and animal-loving teammate on the Alpha Team. She is dating a local nonspell blonde-haired named Edward. She has brown waist-length wavy hair and eyes. Her familiar is a white panther named Sheeba. Her human name/nickname is "Sparrow" to conceal her true identity as both a princess from another world and her bestial curse to turn into a horned, fanged monster whenever she is furious, saddened or scared.
The episode is up on YouTube with the original French-language audio (couldn't find an English-dubbed version, I'm afraid), and you can see Princess Gloria transforming from her monster form into her human form around the 22:20 mark. You can also see the antagonists imprisoned behind bars in the same scene.