Though for what it's worth, did we EVER encounter another shapeshifting alien that wasn't a Founder?
Oh, yes, plenty. Here's a list of about 30
, though it includes some incorporeal superbeings like Q, some that were just illusion-casters like the salt vampire, and some who used technology to change or disguise themselves. But I'd say there are at least nine corporeal species other than Changelings that could change their shape naturally: the Wraith from ENT: "Rogue Planet," the unnamed shapeshifting thieves from ENT: "Two Days and Two Nights," the Antosians from TOS: "Whom Gods Destroy," the Vendorians from TAS: "The Survivor," the Chameloids from TUC, the space jellyfish from TNG: "Encounter at Farpoint," the Allasomorphs from TNG: "The Dauphin," the coalescent organism from TNG: "Aquiel," and Steth from VGR: "Vis a Vis." (And the genetically enhanced Suliban, but that falls a bit outside "naturally.") Plus some that might have been using either shapeshifting or illusion projection, like the Devidians from TNG's "Time's Arrow" or the abductors from TNG: "Allegiance."