It’s been awhile. But I will never, ever stop. Until I get to z, then by the laws of our alphabet, I have to.
So what’s up with D? The best spot to see the evolution of the letter is in the alphabet gif I always show you. As you can see, D was also backwards in early Latin, which makes sense considering d (the lower case) faces the opposite direction. The Romans, who used Latin, took their alphabet (but not the words) of some of the people they conquered, the Etruscans, who for the “d” sound, used that backwards D. And the Etruscans, sometime around the sixth century BCE, took their language from the Euboan Greeks, who used Delta, capital Δ and small letter δ, which don’t look all that different from what we use.
The Greek language is old, existing for at least seven hundred and fifty years before what we call the Common Era or Anno Domini and was the first alphabet in Europe. It was adopted from the alphabet of the Phoenicians, also the parent language of Hebrew, who had the D sound symbolized by the word for door, daleth.
Go back to the gif. The Phoenician symbol looks like a scalene triangle or a tilted flag, not really a door. If you go back even further, to Proto-Sinaitic, it looks even stranger, more like a fish (for a better illustration, go here and scroll down a bit). Talk about ancient—this language is from 1700 BCE. Unlike their symbol using Egyptian neighbors, they used consonants instead of words, adopting the Egyptian daled/daleth (door, specifically a tent door) for their d sounds.
As to why it looked like a fish, I have no idea. This was something like thirty five hundred years ago. Details are a bit sketchy. Delta is also known as a river mouth, but that wasn’t until relatively recently, in the sixteenth century, so that isn’t the reason. I guess we’re going to have to chalk that one up to a mystery, at least for now.
Ancientscripts.com for information on Etruscan, Greek and Proto-Sinaitic
The Origins of the Alphabet gif