What are the normal black responses to rookside openings? Does the Rookside opening have a name? It is not very popular, I believe so I'm wondering what the best defense could be.
One of the foremost principles of chess is to control the center squares.  Side pawn openings do not do this, which is why they are unpopular.  My recommendation would be to simply control the center with your pawns and develop normally.

An old chess saying goes like so: "When your opponent attacks on the side, strike in the center".  The logic behind this is when your opponent is occupied on an insignificant region of the side, they will be unprepared for a direct assault in the most important section of the board - the center.

There is no need to treat these kinds of openings differently - just develop your pieces and control the center like you normally would against a normal opening.  Then once you reach the middlegame, you will be far better prepared for the inevitable struggle due to having more pieces out and in much better outposts.

If you would like to read a further discussion on this topic, here is a link to a chess.com forum on it, where various chess players throw in their two cents:

https://www.chess.com/forum/view/chess-openings/the-rook-pawn-opening

If you want a cheap book on this, here is a link to Chess Grandmaster Kontronias' book on "Beating the Flank Openings".  It only costs about $9:

https://www.amazon.com/Beating-Flank-Openings-Vassilios-Kotronias/dp/0713477814