Basically, Arioch, how an animal/creature reacts is not random - there are reasons whether we learn of them or not. They all have motivations, fears, instincts, intellectual curiosity (to one degree or another), and all that can change with the time of day, season, and if they have already eaten. (The latter primarily for carnivores, of course.)
So I start with an understanding of both the animals/creatures around, and decide if I want something a little weird or gonzo. If so, then I will - generally before hand, as most of my "random encounters" are rolled well ahead of time so I have some time to make them interesting and not feel like random encounters, but a natural part of the flow of the adventure - roll a random reaction taking into concieration all the variables I realize (I am sure there are many I miss). That will determine the reaction; so you can have a bangled tigeress somehow come to think of one of the PCs as her cub and treat him appropriately. (Going for gonzo here, of course.)