I started Early - Took my Dog
And visited the Sea
The Mermaids in the Basement
Came out to look at me
And Frigates - in the Upper Floor
Extended Hempen Hands
Presuming Me to be a Mouse
Aground - upon the Sands
But no Man moved Me - till the Tide
Went past my simple Shoe
On a literal level a mouse may travel on a frigate and escape when in port. Hence the poet represents this abandoned — aground —  creature. 
 However, there is another interpretation, a very subtle yet strongly feminist sentiment expressed in this one line. “Presuming” implies that the persons forming this judgement were doing so without any information to back them up, and thus probably made the judgement in error. They are presuming that she is a mouse – that she is small, weak, and at the bottom of the food chain. This is pretty much the view society had of women at the time. They were seen to be the weaker sex, with no real strength of their own. However, what Dickinson is clearly stating here is that just because others may assume this of women, does not make it so.
I Started Early - Took my Dog