I noticed that my sneaky assassin doesn't get nearly as close to animals as people before being spotted. Is this my imagination or is it really part of the game? It makes complete sense to make it harder to sneak up on a wild animal than a person. I've actually noticed this with monsters as well - chaurus in particular, but not so much with frost spiders.

I also noticed that dragons can spot my little Khajiit from a mile away (that's when it's time to run). I'm not sure if nighttime improves the chances of remaining hidden but I haven't noticed anything dramatically different.

Sneak is at 100 and fully perked. She wears no armor and walks with muffled shoes (not boots).

It depends on the creature. Wolves and frostbite spiders are easy to sneak around. Dragons and bears are the most alert creatures in the entire game and thus are really, really hard to sneak up on (the bear in the cave at Helgen is actually hard-coded so it doesn't instantly spot and maul you).

yeah just depends on the type of animal and how heightened their senses are. animals that are hunted such as deer and elk are hard to sneak.