The Dog
Scholars have said that seeing a transformation into a dog in a dream indicates negative behaviors. If one sees his head as a dog's head, he will treat people with foolishness and and meanness, and if one's body transforms into a dog's body, it signifies that he engages in foolish and base actions.
Per Ibn Sirin's Muntakhab al-Kalam fi Tafsir al-Ahlam
Note: These are traditional scholarly interpretations (ijtihad) drawn from a specific classical source — not a binding religious ruling, fortune-telling, or a prediction of the future.