+1 for flavor explanation

@GaelL I was going to make a tastes like chicken joke in there...

Another consideration is that a lot of aggressive snakes don't always move with their entire body on the ground. Many have their front halves lifted in order to strike. Prone may force their body to the ground thus disadvantage on attacks (harder to strike from that position)

@eimyr Or maybe, their lifting their heads/partial body up which restricts their movement and limits their attacks as they are 'out of position'. Ultimately, how you want this to appear is up to the DM - get creative and fun :)

"Traditionally, we imagine prone as lying on one's stomach" Crap, I always imagined people who were prone as lying on their back. Everything thing I thought I knew is wrong!

If you look at the effects of being prone on a human, one of the significant ones is that we move much slower. A snake generates its speed from having its body in contact with the floor, so if it raises the front half of its body for better visibility, it will move much slower. You could quite easily house-rule it so that you go by the effects of being prone, rather than trying to apply the human method of being prone to non-humanoids

Thanks for the edits @psmears!

Just being a logic Nazi but "...lying on one's stomach - which seems to be at odds with a snake's normal movement" how so? Laying on it's stomach really doesn't seem at odds with it's normal movement at all.

@Shufflepants that's supine, rather than prone. I'm sure this doesn't invalidate the rest of your knowledge.

But your statement, "..or something else mechanically that prevents a creature from getting the condition", seems to specifically imply snakes.

@Octopus Not at all. I was implying things like Hover for a flying creature.

@Octopus Sorry, ignore that last one. Was basically just hedging my bets against other mechanics for class abilities, spells, etc.