By and by I was close enough to have a look, and there laid a man on the ground.
I thought it would be correct to say: There lies a man on the ground. (now) There lay a man on the ground. (before)
If laid is used here as Past Tense, therefore the Present Tense is: There lays a man on the ground.
How to understand this usage?
Twain's use is non-standard, I suspect this is part of the conceit of the novel. It is presented as if it is a transcript of spoken English, and this kind of error is common in spoken English. In speech, many people use laid instead of lay as the past simple of lie: "He laid down and went to sleep."
There is a separate issue of the syntax.
This is a "locative" inversion. English allows for the subject and verb to change places when a phrase indicating "place" is put at the front of the sentence:
Jim sat on the chair (no inversion)
On the chair sat Jim (the subject has inverted with the verb)
Here the locative phrase is "There"
A man laid there, on the ground (no inversion)
There laid a man on the ground (inversion)
