I noticed something odd during Ballphin testing, and now I figured out more about it. Ballphins created by Ballphin houses have a chance to create a "ballphinpod" when they spawn. This seems to happen every time unless they become distracted before they can create the pod, or if there is already another pod in range.
This seems to happen at worldgen, when spawning them through console, and when crafting them yourself and building them.
Edit: The pod creates a herd, which will rapidly mate and duplicate in the first season.
Using console commands to count prefabs "ballphin" and "ballphinpod", as well as making it the next day through the first mild season, it becomes clear that the Ballphin population rapidly explodes.
In the following example, the game starts with 68 Ballphins belonging to 68 Ballphin Palaces, while 48 Ballphin Pods are also present.
After a single Mild Season, the Ballphin count goes up to 492, while the Ballphin Pod count also rises to 65.
Further Edit: I tried to replicate my results every time and couldn't. The number of Ballphins was very large, but not what I had expected. The population explosion would happen near the end of Mild Season, after Jellyfish Migration, and continued from there. I think I must have done more in the first test to influence the results than I thought, so the repro-steps won't give the exact same result. You have to save and quit after the Ballphins create the Ballphin Pods, then go back to try the test.
1. Create World. 2. Countprefabs ballphinpod and ballphin. 3. Create ballphinhouses. 4. Check ballphinpod count afterwards. 5. Save and Quit. 6. Reload Save 7. Advance day cycle while checking ballphin and ballphinpod counts.