When the players pass around a Ring of Spell Storing into which a Paladin has cast "Find Steed" into, so everyone gets a "Found Steed", what does the DM do? This is the sort of thing that differentiates the Ok DMs from the Really Good DMs. The former might throw up their hands, saying "What ?!!!? This breaks the game! I can't allow this! I need to either find something in the rules that prevents it, or change the rules to prevent it". However, the good DM says "Hmm...ok. You do that. Now, for the consequences."
So what are the consequences for when the whole party gets a "Found Steed"?
Well, think about this for a moment. What is the spell "Find Steed"? It is a class feature for a Paladin, set up to work through a spell. A paladin is a holy warrior that gets certain perks in exchange for serving the cause of his/her deity - being their right hand, the executor of their will. One of those perks is a "Found Steed" - a celestial/fey/fiendish spirit that, sent by the deity, assuming the form of some creature, and has intelligence (>=6) (thus a sentient NPC), with a telepathic link (with within a mile). Sooo, a non-paladin wants a "Found Steed"? Do they now? Ok. But, that comes with some baggage. The Found-Steed is a package deal - with a whole set of obligations. The obligations balance the perks. The non-paladin gets the steed, _and_ then also gets the baggage - they need to ACT LIKE A PALADIN OF DIETY X to some degree - and this is perfectly fine. How does this play out? Well, the Found-Steed will insist its owner serve the interests of deity X, live up to their standards, etc. The steed knows what the owner is thinking. So, suppose the owner steals something from a merchant. The steed, having been sent from a LG deity is itself LG. Because of the telepathic link, they know exactly what the owner did, and what they are thinking. The steed might refuse to take the owner anywhere until they go back to merchant and atone for their misconduct. The steed is a sentient NPC, and thus is ultimately controlled by the DM. The steed can rebel or refuse to serve the PC if the PC acts in a way incompatible with the wishes of the deity that sent the spirit that became the steed. The PC can not hide _anything_ from the steed because of the thought-sharing that goes on with the telepathic link (thats how I run the telepathic link - its literally like two minds, each knowing the thoughts of the other). This can become very much a "be careful what you wish for - you just might get it" scenario. Bottom line - if the PC wants a perk of the Paladin class - a Found Steed - and they want that steed to actually do as they ask and help them - then the PC must act like a paladin of the deity of the paladin that cast the Find Steed spell into the ring of Spell Storing (or Ioun Stone of Reserve).
So how would this play out in the end? Each PC is either going to have to adopt the same standards of conduct as the paladin that did the casting - in which, ok, fine, that is the price of the perk. Or, the PC will quickly get fed up with her/his Found Steed, and humorously, decide "screw this!!! I want a normal steed! One that won't judge me and lecture me all the time! Argggg!!!". Ha ha ha
See? No need for the DM to fret. These things work themselves out.