I was wondering this as well along with the new animal (rabbits) because on my current world I've been playing for about 6 months or so updated I couldn't find a single rabbit ANYWHERE! Even searched the snow biome near me as well. I did start a new world and saw many rabbits within the minutes I created it. I'm not sure how the update applies to already discovered places, seems like there may be an issue.
The Meaning of Life, the Universe, and Everything.
I'm almost 100% positive that the area needs to be undiscovered to spawn anything new. I have found 4 monuments in my "old" world, problem is no guardians spawn so my monuments are basically empty. I have found, and captured 1 rabbit
They do spawn in unexplored parts. In my old world I had only discovered one maps worth of my world. I went into a different map and found a monument straight away with bunnies not too far away from it too.