There’s plenty of options, and honestly the very idea of “Low” population on many of the Classic servers is actually laughable. Back in 2008, a WoW server was considered “Full” if there were 10K characters logged in at once. With the tech that they have now, servers can support many more players now than they did 14-18 years ago. If these servers existed in Classic, they’d have been classified as being High/Full, but with the current server tech they are listed as being “Low.”
There’s the topic itself on the matter, as well as Asmon looking it over and breaking it down. I’ve linked it to the relevant portion, but in the event it starts at the beginning, just go to about 9:30 in the video.
With all that said, if your only concern is the mount and you have no intention to continue in Classic Wrath after hitting the mount requirements, then just pick ANY low pop server. While there is normally a requirement of having a level 55 character in order to create a DK, the first one has no such restriction as Blizzard knows some folks were specifically waiting for Wrath. Only the two fresh start servers have a restriction in play.