He bought a rotten fish. Here 'rotten' is a/an -

Rotten is a participle.
- উপরের বাক্যে Root verb এর past participle ‘rotten” is used and its work as an adjective also.
- So, it is a past participle.

Present participle (Verb+ing act as an adjective) such as:
- A sleeping dog seldom bites.

Source: Advanced Learner's Grammar and Composition by Chowdhury and Hossain