[Pineapple924]

Supposedly (as in random twitter users have started claiming): The ViStamps we've seen so far are all based on Heisei Riders and animals because the person at Fenix who invented them (the guy with the glasses and wild outfit) is in-universe a Kamen Rider fan, with the Heisei Riders in general being his favorite.

Assuming that idea turns out to be true, then the in-universe reason for the random animal and rider pairings (other than Revice being the 50th anniversary) is that the ViStamps were supposed to be like the ProgRise keys, but then the inventor went ahead started cramming in Rider designs/motifs too because he thought it would be cool (akin to how Ryoma added sound effects into the sengoku drivers and lockseeds).