I've doubts there's an intentional pattern. Blackbuck also seems maybe a little too popular to be free.
There is at least one DLC with relevance in the year leading to each anniversary, (Africa before the lemur, Europe before the Deer & Tropical before the Peccary), so my guess is simple stuff they considered briefly for a DLC and kept in mind for later.
By that theory, it'd be Eurasia or Barnyard relevant, maybe stuff like Bharal, Himalayan Tahr, Goitered Gazelle, or Banteng from Eurasia, and stuff like Zebu, Kunekune, African dwarf goat or Horses/Clydesdales from Barnyard.
Thinking about it, horse drawn carriages/trams are tied to Clydesdales, so that'd be an interesting side feature/object if they got added.