The Phantom design I came up with for Astral Chill is one I'm really proud of, and I'll probably do another post about all the animals I was looking at, but I thought I should just go ahead and start off with one I think is a little obvious.
The best alien designs are ones designed for harsh environments, and I think that Danny would agree with that, so when he unknowingly modeled himself after the monsters in his favorite films, he became what would be an apex predator on another world. I pulled from aliens that I think a lot of us know and love-xenomorphs from Alien, which have so many subspecies that on my journey to make sure that Phantom isn't just a "xenomorph-like" I realized that most things I could try would end up looking like one and that isn't necessarily a bad thing.
The main bit that the xenomorphs helped me with was figuring out if it's possible for him to move in three different ways on the ground depending on what he needs: quadroped, biped, and climbing with just his hands to take off for flight. I was also thinking about just how human he could look with such a long tail screwing up his posture.
I kept going back and forth in the planning phase figuring out if Phantom would be better quadropedal or bipedal, which would be more alien and frightening. Ultimately, I decided that since he's a ghost, he doesn't have to follow biological rules and he can switch between them whenever he wants. I keep emphasizing that he's gelatinous and boneless so that it makes some sense why he's able to break those rules. He can comfortably walk on his legs, move on his hands and feet, and just use his arms, all by choosing to and having his body accommodate that. It's the same with his size: his smallest would be around the size of a German Shepherd, while his largest could be a car. It depends on how much energy he has and if he's trying to look human or feels threatened. It's a safe bet that if he's with someone he trusts, like Tucker or Sam, he's just human sized, while his parents will make him much bigger.