Alas! and am I born for this,
To wear this slavish chain?
Deprived of all created bliss,
Through hardship, toil and pain!
How long have I in bondage lain,
This is clearly a metaphor to the restrictions that slave life has on slaves. Horton was not really chained up to a fence post or anything, in fact, he was delivering items to the university of North Carolina and  one can not be chained and free at the same time. 
 Info on his life as a slave: 
 

On Liberty and Slavery