I'm currently watching Star Trek: Deep Space Nine and as all you know there are many appearances of shapeshifting/changeling characters like the Founders and Odo.
I watched Star Trek Voyager 5-6 years ago, and I think I remember the crew encountering some shapeshifting species as they were travelling home?
So, are there any other changeling species in Star Trek besides The Founders?
Yes.
In the TNG episode "The Dauphin", there were shapeshifters native to Daled IV. The future monarch of the species as well as her bodyguard/protector had the ability to assume humanoid and animal forms.
In the TNG episode "The Survivors", we were introduced to a Douwd, a powerful being capable of either changing or disguising its appearance. It was able to dematerialize and move around without the aid of a transporter, so its control over its physical structure must be near total.
In the movie Star Trek VI: The Undiscovered Country, inside the Rurapenthe prison complex Kirk and McCoy encountered a shapeshifter that called itself a chameloid. It could assume humanoid and animal forms.
In Star Trek: Voyager, creatures capable of full mimicry of humans were encountered on the class Y "demon planet". These creatures were featured in the episodes "Demon" and "Course: Oblivion".
In Star Trek: Deep Space Nine's episode "The Alternate", another Shapeshifting species was encountered. This one could retain shapes, but it was more of a powder, so its abilities were limited. This species may be related to the Changelings.
The Wraiths in Star Trek: Enterprise's episode "Rogue Planet" were slug-like by default, but were very capable shapeshifters.
Some of the Suliban from Star Trek: Enterprise have shapeshifting abilities, but this is a product of genetic engineering.
Besides the species that are naturally shapeshifters, some humans also can shape shift through mental discipline. In the TOS episode "Whom Gods Destroy" mental patient Captain Garth repeatedly alters his appearance to mimic other humans. And in the episode "Mudd's Women" crones convert themselves into beautiful young women apparently via the placebo effect.
If you define 'changeling' as being a shape-shifting species, possibly the largest one we see is in TNG 'Encounter at Farpoint' - the Spaceborne Entity. We see one transform itself into
Farpoint station
and another in the form of a massive starship.
The beings in Star Trek: Voyager are made of a biomimetic compound. They came from the Class Y planet on the episode Demon. They actually appeared on the episode Course: Oblivion. They had no knowledge of what they really are and it is unclear if they had any conscious ability to assume new shapes or where actually shapeshifters at all and not some type of autonomously self-assembled entities.
In terms of other shapeshifters the memory alpha link can detail those, the majority appear to be able to transition from one discrete morphology to another, with the exception of the Changleings who are natively amorphous. Shapeshifters
I believe the shapeshifters from voyager your looking for are the Undine, otherwise known as species 8472 by the borg. voyager episode "in the flesh" is entirely about how they set up a complex to masquerade as humans to infiltrate and the federation, as they deemed humans/the fed their greatest threat and deemed the borg irrelevant. Undine There form of shapeshifting is even harder to detect then the changlings, which was dam near perfect as it was.
The Memory Alpha page that Richard linked to has a section on "Non-corporeal to physical shapeshifters", which includes, for example, the Organians. They seem to have overlooked Trelane (The Squire of Gothos) who was presumably some sort of energy being (inasmuch as that's what his parents were) but could assume humanoid form.
In "The Survivor" an episode of the kind of sort of semi-canon Star Trek: The Animated Series an multi tenticled Vendorian was encountered. Vendorians can take the shape of other life forms, and this particular one even turned himself into a functioning machine while that machine was desperately needed!
KIRK: There used to be only two examining tables in this room. Now there are three. SPOCK: I just realised that. KIRK: This is a vial of Orientine acid, Winston. It will burn through almost anything but this crystal. If you've never seen it work, I'll demonstrate on you. (The table transforms into the alien) MCCOY: I saw that, but I don't believe it. SPOCK: A Vendorian, Doctor. Their planet is quarantined, and few people ever do see them. Their ability to rearrange their molecular structure into anything with the same general size and mass and their practise of deceit as a way of life puts them off limits. KIRK: Mister Spock, get a security team down here on the double.
and later:
KIRK: Good work, Mister Scott. That deflector shield went up just in time. SCOTT: But sir, I haven't finished repairing the shield. I told you it'll take hours, and it will. SPOCK: Interesting. KIRK: Then what was? Winston. SPOCK: It is not outside the realm of probability. If he could rearrange his own atoms to become an examination table, one would have to assume he could become a deflector shield.
1http://www.chakoteya.net/StarTrek/TAS005.htm
When Garth of Izar was severely wounded near Antos IV, the Antosians taught him the process of cellular regeneration to heal himself, and Garth later used it to change his shape and impersonate other beings (Star Trek: The Original Series "Whom Gods Destroy").
KIRK: How does he manage to change form at will? CORY: The people of Antos taught him the techniques of cellular metamorphosis to restore the destroyed parts of his body. By himself, he later learned to use the technique to recreate himself into any form he wished. The first time we knew about it was when a guard, seeing what he thought was me in Garth's cell, released him.
1http://www.chakoteya.net/StarTrek/71.htm
So Captain Kirk should have kept an open mind and not been certain that Cameloids were a myth until Star Trek VI: The Undiscovered Country.
MARTIA: I'm a chameloid. KIRK: I've heard about you. 'Shapeshifters', I thought you were mythical.
1http://www.chakoteya.net/movies/movie6.html
So that might (depending on various factors such as the canon status of TAS and whether cellular regeneration is innate to all Antosians) possibly make as many as three other canonical shape shifting biological (non energy beings) species in Star Trek before the changelings in Star Trek: Deep Space Nine!
