As the titular space station's head of security, Constable Odo (Rene Auberjonois), a Changeling, was an integral part of Star Trek canon by DS9, and they proved to be both a fascinating species and a worthy foe for the Federation. Possessing no solid form, the Changelings were capable of assuming any form, and this metamorphic ability led to distrust and often violent reactions from the many species they encountered.
The Changelings responded to the violence and distrust they met at the hands of the "Solids" by retreating to a planet where they existed as a hive mind - the Great Link. They plotted to restore order to the galaxy, creating the Jem'Hadar and enlisting the Vorta ahead of DS9's Dominion War. The mystery of the Changelings and Odo's origins defined his story arc in the early seasons of DS9 and once he discovered his home world in season 3's "The Search" he and the viewers were introduced to all manner of Changeling characters. Here's every named Changeling who appeared in DS9.
Odo
Odo was notable for his rejection of the Founder's belief that humanoid life should be subjugated to restore order. Originally discovered adrift in the Alpha Quadrant, Odo went on to form meaningful relationships with the Bajorans including Major Kira Nerys (Nana Visitor) and Mora Pol (James Sloyan), the Bajoran scientist assigned to him. Odo even married Lwaxana Troi (Majel Barratt) and became an unwitting pawn in Section 31's genocidal plot against the Changelings, when he was infected with the deadly Morphogenic virus.
At the end of the Dominion War in Deep Space Nine, Odo returned to the Great Link to aid the peace process by teaching the Changelings about the merits of the humanoids. As Rene Auberjonois sadly ed away in 2019, it's unlikely that Odo will return to Star Trek in a substantial role. However, the unique character's teachings may have inspired a new relationship between the Changelings and the Federation in Star Trek's future. As a Changeling appears in Star Trek: Discovery season 4, it's clear that they continue to exist in the galaxy nearly a century later.
The Female Changeling
The Female Changeling (Salome Jens) had a complex relationship with Star Trek: DS9's Odo. She played on his feelings of isolation, and his attraction to Major Kira in a multiple attempts to lure him back to the Great Link. However, despite her manipulations, Odo still felt a kinship with the Female Changeling and cured her of the Morphogenic virus by linking with her. At the end of the Dominion War, the Female Changeling was imprisoned for the multiple war crimes she had committed. Her whereabouts are currently unknown in the continuing on-screen canon, but there's lots of story potential in a DS9 season 8 that revisits her and the Dominion.
The Infant
In DS9 season 5, episode 12, "The Begotten", Odo buys an ailing infant Changeling from Quark (Armin Shimmerman). Sentenced to life as a solid by the Great Link, Odo attempted to reconnect with what he'd lost by teaching the infant how to shape-shift. Tragically, the infant wasn't strong enough to survive, but at the point of death, restored Odo's abilities by absorbing itself into him.
The Martok Changeling
The increased hostilities between the Klingon Empire and both the Federation and the Cardassian Union was a strategic move by the Dominion to destabilize the Alpha Quadrant. To do this, they installed a Changeling who posed as General Martok (J.G. Hertzler), a key advisor to Chancellor Gowron (Robert O'Reilly). Martok's duplicate led the Klingon Empire into conflict with the Federation, necessitating Worf's presence on DS9 from season 4 onwards. The real Martok was held in a Dominion internment camp, alongside another key DS9 character, who had been replaced by a Changeling.
The Bashir Changeling
When Garak and Worf discover the real General Martok in the Dominion internment camp in DS9 season 5, episode 14, "In Purgatory's Shadow", they also discover the real Julian Bashir (Alexander Siddig). Bashir had previously been replaced by a Changeling in DS9 season 3, but only for a brief amount of time. The implication of Julian's capture on Meezan IV was that the Bashir audiences had been watching over the previous weeks was an imposter who had been plotting against the Federation. The Bashir Changeling was destroyed by DS9's starship, the Defiant, when he attempted to destroy the Bajoran sun.
The Krajensky Changeling
Bashir had previously been impersonated by the Krajensky Changeling, who had infiltrated DS9 during Benjamin Sisko's promotion ceremony. The Changeling impersonated a Federation ambassador who sent Sisko and the Defiant crew on a mission to the planet Tzenkathi, where the Dominion intended to use the Starfleet vessel to trigger a war. After Krajensky was revealed to be a Changeling, he fought with Odo in the Defiant engine room. Odo killed Krajensky during their confrontation, giving him the dubious honor of being the only Changeling to ever kill one of their own.
The Lovok Changeling
Five episodes earlier, Odo was informed of the law that forbade Changelings from harming one another when he encountered a Changeling posing as the Romulan Tal Shiar officer Colonel Lovok. The Changeling used his position to attempt to weaken both the Cardassian Union and the Romulan Empire. Luring both the Romulans and Cardassians into Dominion space under the pretense of destroying the Changeling home world, Lovok stood back as both fleets were destroyed by the Jem Ha'dar.
Laas
Odo met Laas (J.G. Hertzler) in DS9 season 7, episode 14, "Chimera", who essentially mirrored Odo's own journey. One of the 100 Changelings sent to gather information on the humanoids, Laas had grown weary and disillusioned with the solids' self-destructive tendencies. While distrustful of humanoids, he didn't share the Founders' belief in domination and subjugation. At the end of "Chimera", Laas left to seek out the remaining Changelings and establish his own Great Link in the Alpha Quadrant, neatly foreshadowing the ending of Odo's Star Trek: Deep Space Nine story.