You could target both yourself and your steed, but you'll have to be careful how you proceed
The question is a very interesting one, and mostly based on a semantic distinction. After all, if time were to speed up for you, or slow down (to the point of stopping) for everyone else, the experience of the two events would be virtually identical.
Fortunately, there are a number of sources of information that can help us conclude that Time Stop "targets only you."
Evidence Time Stop normally targets only the caster.
1.) It has a range of self
Now, there are certainly spells that have a Range of self which target more than one thing. Wish springs to mind, because it can create any number of effects that explicitly target something or someone other than the caster (e.g. it could be used to cast Fireball). Also, some spells like Blinding Smite (which has a range of self) explicitly mention a second "target" in their text, which means they do not target only themselves.
However, Time Stop not only has a range of self, it does not refer to anything else in its text as a "target" of the spell. While not conclusive, that's good evidence that it might be able to also target a found steed.
2.) There is consensus that it targets only the caster
There is a very related question on this stack: "Can a creature take turns as normal if they are inside an Antimagic Field while another creature casts Time Stop?" This cuts to the heart of the issue, as it is essentially asking whether other creatures are under the effect of the spell, or if the spell is only changing things for the caster.
In that question, there is consensus amongst the higher voted answers (like this and this, with more than 10 upvotes at the time of this posting) that the spell Time Stop is influencing the caster, not the people around them. While consensus is not firm evidence in and of itself (Galileo was right even when the majority was wrong), it does help our argument.
3.) Jeremy Crawford has given clarification that can help
The arguments in the higher voted answers linked above in point #2 rely on the idea that the caster is taking multiple turns simultaneously (within one turn), rather than everyone else doing nothing on their turns because they are frozen. This perspective is backed up by unofficial designer commentary. In a tweet, Jeremy Crawford stated:
Legendary Actions can be taken only at the end of another creature's turn, so they can't be taken while you're under the effect of the time stop spell, which involves you taking multiple turns in succession.
This indicates that other creatures are not taking multiple turns but doing nothing: rather the caster of Time Stop is taking multiple turns instantaneously. This wording implies that Time Stop is influencing only the caster: otherwise, other creatures’ turns would still be happening (they just would be doing nothing during those turns because they were immobilized).
Crawford has also given explicit (though unofficial) definition to the criteria of spells that are shared with a Found Steed:
Find steed: a spell you share with your steed must have a range of self (without an area) or target only you.
Time stop unambiguously has a range of self, without an associated area (unlike the spell Thunderwave which has an area of "Self, (15-foot cube)"). And the above points of evidence are helpful in determining that it also has no additional targets. So it seems to fit this (unofficial) criteria.
None of these points on their own are definitive: each is based on subjective or qualified evidence. But taken together, they make a fairly consistent argument that Time Stop targets only the caster, and thus could also target the caster's Found Steed.
So... why do I need to be careful?
Let's imagine that your bard character cast this spell, targeting both themselves and the Found Steed. Both of you would then be under the effects of the spell, and have to obey its rules. For example (PHB, p. 283, bold added):
This spell ends if one of the actions you use during this period, or any effects that you create during this period, affects a creature other than you or an object being worn or carried by someone other than you.
Now, a separate question arises: does this prohibition mean that the caster and steed's actions aren't allowed to affect each other? That is a bit less clear. One could make the argument that the "you" in the above sentence is meant to refer to the target of the spell, so both Caster and Mount are included, and thus are allowed to affect each other. On the other hand, a DM could rule that the "you" in that sentence refers to each being separately, and as such is a prohibition that applies to each.
It is also unclear whether the word "action" here refers to the Action which every creature can do once on their turn, or a more informal sense of the word. At first, it might seem obvious that it is intended to be the former: after all, the word "action" is used quite carefully in this game. However, it's possible that it is meant as the latter (and movement also is not allowed to affect other creatures): for example, a person casting Time Stop could try and steal an enemy's sword as part of their movement, arguing that they can interact with one object for free on their turns and that this doesn't use an action: but it's likely that a DM would overrule that idea, claiming that the influencing of an object being worn or carried by someone else is clearly meant to stop the spell.
As such, there's an argument to be made that the simple act of a mount carrying the caster around is "affecting" a creature other than itself (as well as the items being carried by that other creature), and thus would end the spell.
But that's not an argument I'd personally make.
Personally, as a DM, I would completely allow the Steed and the Caster to influence each other (i.e. allow one to ride the other) without ending the spell. It mostly would just give the caster extra mobility during the Time Stop, and is empirically cool. And the ambiguities in the language are vague enough that it could be ruled either way. As such, I'd trend towards RAF.
Beyond that personal ruling though, you'd need to check with your DM. If that DM feels that the main intention of the prohibition in Time Stop is to prevent you from influencing those creatures (or objects) frozen in time, then they likely will allow you to interact freely with your mount during Time Stop, and give aide to each other. On the other hand, if they read the rules very literally, they might conclude that your mount must not affect you, nor you your mount, at all during Time Stop, or it will end for you both. As such, though it seems clear that Time Stop can target both a caster and their Found Steed, a caster might want to choose to target only themselves for fear of ending the spell too early.