Can a dog be obese?

Obesity is a condition, not a virus or disease, so any animal can be obese.  The definition of this condition is when the animal is more than 20% over its ideal weight, which applies to humans and non-humans alike.

Now, medically, there's some question and debate about what an ideal weight is depending on a person's body type, genetics and frame.  It's pretty easy to say there is no one size fits all definition of who is obese and who isn't.  But the 20% rule of thumb is pretty good all the way around.  More specifically, a BMI (Body Mass Index) of 30 or above is also considered obese by the National Insitute of Health.

So yes, the dog or cat can be obese, and as pohnpei pointed out, many, many pets are.  Perhaps because the pets tend to have the same activity levels as their owners.