Prior to his introduction, Thorax appears briefly at a distance in the final shot of The Crystalling - Part 2, flying towards the Crystal Empire.
In The Times They Are A Changeling, Thorax appears pretending to be Spike's mirror reflection, but he is exposed when he does not copy Spike's exact movements. Spike is scared of Thorax at first, but he starts to warm up to him after Thorax saves him from falling into a deep crevice. Thorax explains that, unlike the other changelings, he wants to make friends, having desired so ever since he saw the Mane Six fight off the evil changelings during the royal wedding in Canterlot.
Thorax also explains that he was drawn to the Crystal Empire due to the love the Crystal Ponies had during Flurry Heart's Crystalling, but he has been unable to sustain himself off of that love because of what he is. Spike, sympathetic to Thorax's dilemma, decides to help him.After Spike fails to convince the ponies of the idea that a changeling can be nice, he has Thorax disguise himself as a Crystal Pony—proportioned similarly to Plaid Stripes, "Pearly Whites", S06E24 Unnamed Pegasus Filly #1, filly Fluttershy, later filly Applejack, and later filly Derpy—named Crystal Hoof. With this disguise, Thorax is able to meet Princess Twilight Sparkle and mingle with the Empire populace. However, his cover is blown when he meets Flurry Heart and the love around her causes him to succumb to his basic instincts. In the confrontation that ensues, Spike briefly hesitates to stand up for Thorax, and Thorax runs off in tears, feeling betrayed.
When Spike returns to Thorax's cave and attempts to make amends, Thorax angrily turns him away at first, but he demonstrates his friendship by saving Spike from the crevice a second time. Spike brings Thorax before the ponies again and expresses through song that Thorax is good and that even changelings deserve a chance at friendship. Twilight, moved by Spike's courage and kindness, accepts Thorax as her friend, and the other ponies follow suit, welcoming Thorax as an official citizen of the Crystal Empire.
Thorax gratefully accepts the ponies' offer of friendship and expresses a desire to learn all he can about friendship and share what he learns with his fellow changelings so they can share love with each other instead of stealing it from others.
In To Where and Back Again - Part 1, Thorax teams up with Starlight Glimmer, Trixie, and Discord to save Equestria from changeling invasion. Here, his wings have become coated in crystal glitter. In Part 2, Thorax is forced to confront his doubts about returning to the kingdom where he was born, and he guides his allies into the Changeling Kingdom when the properties of Queen Chrysalis' throne disables their magic.
Near the end of the episode, it is revealed that Thorax's wings changed as a result of him sharing love with the Crystal Ponies. When Chrysalis tries to absorb that love from him, he follows Starlight's advice and shares it with her willingly, causing a blast of magic that metamorphoses him into a new colorful form. After Chrysalis is defeated and chased off, Thorax becomes the new leader of the changeling hive.
Thorax and the reformed changelings appear at Starlight's graduation ceremony in Celestial Advice. Twilight also considered sending Starlight to the Changeling Kingdom to help Thorax teach the changelings about friendship. In Triple Threat, Spike accidentally invites both Thorax and Princess Ember to Ponyville on the same day. During his visit, Thorax confides in Spike that he is dealing with a renegade group of changelings in his kingdom that refuses to adopt the hive's new ways. When he overhears Ember getting angry at Spike, Thorax transforms into a bear to defend him. After the misunderstanding is cleared up, Thorax helps Ember to be sensitive to the feelings of her subjects and to be more open with her own feelings. In the end, he takes part in a ceremony to unite changelings and dragons in friendship.
Role in the series
Thorax has a possible role in the show. Turns out, he has a large bounty on him by the Villain League, especially his former master Queen Chrysalis. Thorax, ever since running away from the Villain League, has been on the hunt by them, and had offered a huge reward to any villain/villain enthusiast from even Mugshotra who can turn him in, hoping that even heroes would do so as well because Chrysalis believed that, after what her kind did, nobody would possibly trust Thorax. Thorax had been trying for years to find the kingdom of Chrysalis' kind-hearted and more peaceful sister Destiny the Changeling, but since the Changelings were always there waiting for him, he was doomed to not have a happy life until Spike helped him get off his hooves. However, this leaves the high risk of the Villain League planning an immediate invasion on the Crystal Empire just to get their traitorous former member.