adding people to lists... spam?

say I have a twitter account linked to an info website. say for example the site is about horses. If I then start following everyone who mentions horses in their tweets, then add them to a list called "horse lovers" or something... is that spam? people might not check to see whos following them, but they usually notice when they're added to a list. is there some rule against this or will it not work for some reason? if I don't use the account to spam out links, will this even be viewed as spam? sorry I'm just starting to play around with twitter so sorry if this is obvious.

The list feature on Twitter is a way of organizing your followers, I don't see any reason why people would complain that you put them on the horse list. Unless, of course someone gets offended by the word 'horse'. lol