Serpents represent what we know from our bodily senses, and the reasoning based on our senses. Since the people of the Most Ancient Church had become more external, they were susceptible to the lure of trusting their senses more than they trusted the leading of the Lord. That was particularly true for the sense of self the people had been given, which is represented by the woman. Eating of the trees in the garden represented taking in desires for good and true ideas from the knowledge granted them by the Lord.

So here, for the first time, we see people, from their own senses, actually questioning the Lord. From their senses they wished to explore the knowledge represented by fruit of the garden, but wondered why they were denied the tree of knowledge.

(Odkazy: Arcana Coelestia 194, 195, 196, 197)

6401. 'And its rider will fall backwards' means a receding from [truth]. This is clear from the meaning of 'falling backwards' as a receding from - a receding from truth; and from the meaning of 'rider' as those subject to false notions received from lowest nature. 'Horse' means those false notions, see immediately above in 6400, and therefore 'rider' means those subject to them, the nature of which meanings has been discussed immediately above. Since 'Dan' means those within the Church who are the kind of people described just above in 6396 and who are accordingly among the lowest in the Lord's kingdom, 'Dan' also means those who hatch false teachings out of false notions and disseminate them. These people's false teachings are also called 'horses', and their reasonings about truth and good are called 'serpents', in Jeremiah,

From Dan the snorting of his horses was heard; at the sound of the neighing of his strong ones the whole earth quaked. And they came and devoured the land and the fullness of it, the city and those dwelling in it. For behold, I send into you poisonous serpents which do not respond to charming; and they will bite you. Jeremiah 8:16-17.