When it comes to correcting pokémon behavior, the most important first step is determining why that behavior is occurring. Your local Nurse Joy may be able to assist you with that step, but other than that, it’s highly recommended that you observe your growlithe closely to see if any part of its barking patterns can give you a hint as to what’s wrong.
That having been said, there are a number of reasons why your growlithe may be barking at nothing. One possibility has to do with the fact that growlithe possess extremely powerful senses of smell and hearing. Your growlithe could be responding to a howl or a bark outside of your home, or it could be responding to pokémon or animal sounds inside. Remember that growlithe are highly territorial pokémon. If any of your neighbors have dogs or dog-like pokémon—including other growlithe—yours will attempt to assert dominance and claim your neighborhood as its own, especially if it’s lived with you at your current residence for a long time. Pay close attention to your neighborhood whenever you take your growlithe for a walk to see if any other dogs live near you.
Alternatively, yes, growlithe also bark at vermin, even if those intruders are inside your walls. Growlithe are hunters, after all, so if your growlithe is staring at a wall during or shortly before barking, it’s trying to alert you to possible prey.
If it’s neither of these things, there may be a plethora of other reasons why your growlithe is barking. Some growlithe bark because they’re scared or anxious, particularly if you’ve just adopted them, if you’ve added new pokémon or people to your home, or if you’ve just moved to a new location. (Some are even afraid of the dark or of storms, so knowing precisely when your growlithe barks may be helpful.) Other growlithe bark because they’re bored and want your attention. Still other growlithe bark out of sympathy. (Growlithe, being highly loyal pokémon, are acutely in-tune with their owners’ moods. If you feel overwhelming negative emotion—say, for example, due to a lack of sleep—your growlithe may be barking as a means of saying, “Hello, trainer! I am here! What can I do for you?” Growlithe unfortunately lack the capability of understanding that they may be the cause of your problem unless you’re visibly upset with them.) In all of these cases, pinpointing the root cause of your growlithe’s barking is simply a matter of observing its body language and the timing of its barks.
Regardless of what the underlying cause is, it’s important to use positive reinforcement with your growlithe. Be firm with your growlithe but never shout or punish it for barking. Rather, reward it for not barking whenever it stops. Knowing the underlying cause just helps you to figure out how, specifically, to get your growlithe to stop barking, whether it’s removing vermin from your walls, soundproofing your home, reassuring your growlithe whenever it’s scared or anxious, or simply being stern with it if it’s simply trying to get your attention.
The Pokédex According to Bill 