My contribution to this topic is that an art isn't necessarily internal or external. Instead, internal and external describe schools of thought on how to teach the principle behind a given art/style. Somethings are best taught using a more overt or external, "put your arm here", "point your toes", "do this as fast as you can", type of approach. Other principles can only be addressed by slower, sensitivity-based approaches, giving practices such as, "stretch out and feel the arm link through the back", "keep your gaze here to help direct your force", "start a coil with the arm and feel it stretch to your belly--use that pulling to turn the body".

Ideally, any mature movement art (not just martial art) concerned with integrating the body in an efficient and effective way will use some degree of external and internal training. Some arts are pretty straightforward and can be learned without needing to feel things like how muscle groups can work synergistically (e.g. mall karate of the 80s, or basic combat drills taught to a pick-up group of farmers-cum-infantry two days before the barbarians invade). But most adults get bored with that and few would spend their lives with technician-level movements that could be mastered in a few weeks or months. That's doesn't mean they are bad styles, just that there isn't much you can do with them in terms of exploration. As an analogy: pop music can be fun but don't look for too much deep meaning in their syrupy lyrics.

The practices that emphasize traditionally "tough" and "external" virtues can have potential for internal development. The legends of boxing (e.g the Sugar Rays) moved a lot like internal martial artists. They just didn't have any explicit traditional practices and language to talk about their "internal" side. And, finally, even taijiquan necessarily has external practices. In fact, it could be argued that "common" taiji (limp limbed new agers hoping to toss balls of qi, anime-style) is quite external since most of the practice is limited to "move arm here", "step here", instead of training the biomechanical connections.

Sorry for the length. Thanks for reading if you got this far. Cheers!