Clue: Water running into shore's rotten – could be 'dead' hippo (8)
Answer: SEAHORSE
Explanation: Definition: 'dead' hippo
**SEA** ("water") *running into*** *(shore) **[anag:*'s rotten*] 
 
 
*The 'hippo' here would be the hippocampus, a legendary sea monster of ancient mythology.