Is pathos a rhetorical device?
Yes, pathos is a rhetorical device, and one that is used effectively in Julius Caesar. Typically, you hear about pathos in conjunction with two other rhetorical devices -- ethos and logos. To illustrate the differences, let's suppose that an animal shelter wanted to advertise that it wanted more animals to be adopted.
A logos approach to this would cite facts and figures. Perhaps a commercial would note the number of animals that have to be euthanized each week because they had not been adopted. Or perhaps the advertiser would cite the fact that pet owners tend to live longer than non pet owners. In an ethos approach, the credibility of the speaker tends to lend credence to what is being said. Perhaps the advertiser would enlist a celebrity to endorse the shelter, knowing that the people would trust what the celebrity was saying.
A pathos approach would tug on the heart strings. The word "pathos" means to evoke feelings of sadness or pity. In a pathos approach, the shelter would, as you sometimes see various animal groups doing, create a commercial with pictures of animals in their cages, looking forlorn (cue the sad music!).
People use logos, ethos, and pathos within speeches to the same effect.