Until a better idea comes along, this Creature Seasonality Table at least allows for some initial collection of some Creature Seasonality data.
Potential Creature Seasonality data correctness issue: When a particular creature is given seasonality later on in future patches its kinda tricky to flush them out (when its not included in the patch notes) ... except by starting with a new fresh and empty table.
Input data:
...: No data available.
Y: Spotted. - The easy one.
N: Confirmed. - Technically this one relies on actual confirmation (dev's or other hard data), the next best thing would be based on statistical data (which is a bit problematic in my view, have to think about that some more).
-: Assumed to have no seasonality based on logical elimination (guessing).
Input options will needs some additional re-thinking. As creature seasonality is not just a bool(on/off) feature, but also a changed probability feature.
For the moment I think adding a "-" after a Y might help. Whereby a "Y-" means it was at least spotted, but at a noticeable lower frequency/chance than is some other season. For most the maximum is probably in summer, but for some it might be a other season (like, based on some Jorb post, wolfs. Which are intended to max in winter).