That's amazing that you mention St. George. He's my patron saint! I have many icons of him in my home and in my office at work. I even have one of him in my car. Here's a great account of his martyrdom, where they tried many times to kill him, finally decapitating him: https://www.goarch.org/-/feast-of-the-holy-great-martyr-geor...
As for the dragon, it represents Satan, and also monsters who are tyrannical, hybrids that are not easily categorized that live in the forest or wilderness, and the foreign or the strange. It also represents time. I also like your friend's answer. Notice how the serpent's tail is wrapped up in the horse's feet, trying to trip the horse up.
The life of St. George has many interesting elements, and also has bleed-over into Islamic tradition as well.
The way I started learning about the language and symbolism of iconography was through watching videos on symbolism by Jonathan Pageau on youtube. He's an Orthodox iconographer and he uses fairy tales and movie symbolism to talk about symbolism in general and to introduce it to modern audiences, and he also does videos about the symbolism in specific icons.
If you have any other questions, feel free to ask here. I don't think HN has a direct message system, unfortunately. If they do and I'm just too boomer to figure it out, feel free to hit me up in DM as well.
Fun fact - I have a few Orthodox Christian icons in my apartment. One of them is of St. George slaying a dragon.
I had a conversation with a good friend about this image and asked - what does the dragon represent?
He said “More”