Source: The Great Interpretation of Dreams (Ibn Sirin)
(By analogy with large, aquatic animals) A hippopotamus may indicate a large man of might and power, or a dangerous ruler. Seeing it in water may indicate looming danger or a hidden enemy.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
(By analogy) It may symbolize a strong and rich man but slow-moving or lazy, or it indicates a matter involving heaviness and difficulty.
Source: Signs in the Science of Expressions (Ibn Shaheen)
(By analogy) It is interpreted as a strong, large man, or an enemy coming from the direction of water. Being harmed by it indicates harm from a strong man.