Add at least a body and one more part, all connected, to save
Your creature is a graph — the same math structure that powers maps, social networks, and the internet. Every part is a node, every link is an edge. The number of links a part has is its degree. When you hit Play, the game reads your graph: leg nodes make it walk, wing nodes make it float, wheels make it roll.
That's real JSON — the language apps use to store data like this.