Tenryu Shen ("family" name first) is a gold dragon. He can, in theory, choose any humanoid form he likes, but he prefers to look like a gold-scaled dragonborn. The idea I get is that it's more natural/comfortable to "wear" the same form most of the time, especially if a dragon intends to stay in that form most of the time. Shen only rarely takes dragon form currently, as he is on an undercover mission trying to track down....
A black dragon. Shen has not given the name of this dragon, but has said that he was close to them in the past. (Gender is elective for dragons, so Shen chooses to be male, it's not clear what this black dragon has chosen.) I know who the dragon actually is, of course, and they too have been wearing a humanoid form for a long time--but they change it semi-regularly, having pretended to be a sequence of people over the years they've been in hiding. They're very close to achieving their goals of conquering the city of Al'Rakkah, not through violence per se, but through both open and covert/illicit economic control, with the intent of making this, one of the richest cities in the world, their "hoard."
Oleander Pierpont Mortcombe is a time dragon the party briefly interacted with. He was stuck on the polydimensional lattice which "wards" the planet the PCs come from. The party did not see him in human form, but presumably he can take one.
An origami dragon statuette the party found. It contains a spiritual dragon inside; apparently, dragons who live a very long time can become spirits, and this one was bound to this statuette for some reason or other. It doesn't seem malicious, but the party hasn't been able to draw forth this dragon yet (they need to know its name first.) Given it is a spirit, its form is almost totally malleable, so it probably could look human if it wanted to, but I doubt it would want to.