Technically, having a horse did. For most of european history.
It's a little more complicated than that, still. Typically there was a certain number of knights who were given their positions for a reason, whether because war loomed on the horizon, defending borders, etc. Not all 'wartime knights' were landed, but frequently they were... which was important because you were expected to provide/upkeep your own arms and armor, including a horse, and maintain a retinue while travelling away from home which some European knights were expected to do for as much as six months out of a given year, visiting other territories/patrolling their own to deal with problems. You were also expected to be able to host your king and his household to a comfortable degree on short notice to avoid insulting his status (which often included feeding the king's retinue of potentially several hundred people, and feeding them WELL at that.)
A poor knight who could not meet these social obligations adequately was often considered low in status, and rarely considered for promotion to nobility... to say nothing of being less able to adequately equip himself and his men in times of war. Not a good thing, you can imagine.