For the movie: The "1 dog years is = 7 human years" rule is downright apocryphal. However, Spot is still equivalent to an adult human male. He happens to be part of a small dog breed.
Not quite horror, but rather squick: The fact that Spot realized that Mrs. Helperman is basically his mother in the fifth episode makes his desperate plan to marry her to continue being human all the more weird and uncomfortable. It's a very good thing that didn't work out.
More like Tear Jerker. Spot mentions that one human year is seven dog years like in real life. That means Leonard will greatly outlive Spot and will live a long life without his best friend.
Given what usually happens to dogs that are overly aggressive (particularly when they've bitten people hard enough to cause considerable physical harm), it's probable that the dog that bit off half of Principal Strickler's thumb was put to sleep afterwards.
