Add terms to the linear model to address these concerns and test the hypotheses. You can add a term for the fixed effect of each animal. This represents a different average response for each animal. You can add a cross-term treatment*animal for the fixed effect of the interaction. This term represents a different treatment effect for each animal. I am concerned that you might not have enough data to estimate everything adequately.
I think that the animals are simply a sample from a population. In this case, treating animals as a random effect makes sense.
Select Analyze > Fit Model. Enter your response in the Y role. Add treatment and animal as effects. Select the animal effect, click the red triangle next to Attributes, and select Random Effect. The launch dialog looks like this: