Years ago, this little white cat named Albert became friends with all the wildlife around, from woodchucks to porcupines, and there wasn't an animal he didn't think he could be friends with--
As he got closer, they got bigger. Much much bigger. He slowed a bit, and they suddenly noticed this strange creature racing toward them. The head doe stared at him as he got closer, and then made that 'hanh' snorty warning sound. He slowed a bit more, and then she stamped at him. Oops.
He turned without breaking stride into a perfect U-turn and headed back to the house, ears back, now, and not happy. Flap wemt the cat door, and baaack up the stairs, to sulk on the bed and think about this.
Since then, and that was a good 15 years ago or more, we have not had a cat that could so much as sit on the porch without causing general irritability in grazing deer.
This morning Toby and Charlie were both on the porch, and there were deer (those teeny deer again) grazing in the same relative area--from a distance of maybe 100 yards or more--neither group had seen the other. The Charlie stood up and stretched and heads came up. He walked down the steps, and Toby followed. There were several snorts and hanhs and the cats turned and looked, and when they did, all four deer turned and flashed the white tail signal, bounding off into the woods.
My theory is, deer, like most animals, pass along information in some way to other members of the herd, and to their young. "this is good to eat, that will kill ya", "these are safe places to feed, but not those'. and apparently, "small unidentified furry creatures with long tails are to be avoided. We don't know why, but they threaten us."