I have a set of elephant whistles on my cage. They are quite impressive, I've never hit or even SEEN an elephant anywhere close to me while I'm driving. Used to have a set on the bike, too, same results.
Deer whistles??? Not so sure about those, I've never heard of any scientific report of them working, or even doing what they claim to do (create high pitched sounds). Then there's the whole issue of what a deer can actually hear.
If there's not too much traffic when you are out at that time, I'd go with some serious candlepower instead. That will hopefully give you more warning time. I have a single 100W high beam that is switched by a relay and powered directly from the battery (through a self-resetting circuit breaker), and it is VERY bright.