They're not the same value. Why do you think they're the same value?
Also, intelligent creatures don't show emotions in dwarf mode unless they're citizens, not just intelligent, so the GUI argument sort of falls flat there, too.
Also, "where it pools into" isn't quite right; these structures are part of larger structures but that doesn't mean their behavior is inextricably linked with said larger structures. Just because unit.status.current_soul.emotions[0] is an emotion contained within unit doesn't mean that unit.animal.vanish_countdown is going to do anything to said emotion.
Because as how I understand it population id in the world data wouldn't work without it, since it needs to read sites in addition to settled areas, creatures that are non-site native are disallowed from leaving and to properly count everything living in every tile across all regions (without having $populationg.reg have anything to do with it) despite screwily as Quietust points out with innacuracies, population_id is already claimed by entity_population at df.history.xml#L340 though the actual population id is marked as 'idx' which im unsure if thats applicable to any code rules.
But intelligent non-citizens (semi intelligent and animal observably) do (or alternatively looking at it, they emulate as non fortress faction citizen units), one such way to provoke a emotional response is to restraint chain for a extended period of time.
Certain individual creatures will become depressed and express emotional states, even the wild animals do the same (personality facets to each creature?), though mostly you can trigger anxiety emotions more easily in animals by exposing [BENIGN] tamed units to predatory wild units with historical names (which are then ignored by local site populations in fighting, though that might be related to my ethic setup, which has its own bug report here about wild creatures only respecting site entity ethical morale engagement rules -
http://www.bay12games.com/dwarves/mantisbt/view.php?id=10059) because that creature killed a member of your fortress, much like the behavior of villagers being struck down with panic in adventure mode. This is not the same as generic animal running fear (which activates when being attacked), the anxious units will just stare though the grate at the carnivorous named animal and rush out to kill it, despite ignoring all other creatures otherwise.
Ill try to get a save up for the future dedicated to this for vanilla and solid science, but they are all things i have seen happening personally and with some effort are re-creatable and reproducible every single time with enough effort to arrange the conditions because its behind the scenes tomfoolery since the mind system was put in.
Sometimes its just common sense, but if it leads you towards where the original type value is defined and hosted (lets say ref-type creature raw, knowing it exists and then searching the directory for all of those entries will lead you to places, from there onwards until you find all the parts that you need and encounter new terms relevant to your query)