The dog, frightened by the thunder, trembled.
Is the bold part adjectival participle?
an adjectival participle*
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The non-finite clause, frightened by the thunder, modifies dog.
The dog, frightened by the thunder, trembled.
No, not an adjective but a past-participial verb in a passive clause (note the passive by-phrase).
The commas mark the whole underlined non-finite clause as a supplement.
It's very similar to the non-restrictive relative construction The dog, who was frightened by the thunder, trembled, wher