What would happen to animal cells if they had cell walls like plant cells?

Well if you mean animal cells having a rigid cell wall like that of plant cells a few things would happen: ... Cells would not be able to transport material in and out of the cell as easily with a cell wall (endocytosis and exocytosis especially would be hampered).

If an animal's body were made of plant cells, the animal would be very stiff and unable to move easily.