Find (greater) steed does not work with magic jar if your steed has no soul
The magic jar spell states (emphasis mine):
Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell's material component. [...]
For the steed to even be affected, it must have a soul. I honestly have no idea whether it does or does not have one; we do have a question on whether a simulacrum has a soul, and I'd just say it is up to your GM whether a found steed has a soul or not.
Assuming magic jar does work; both souls enter the same container
Assuming your steed does have a soul, it would enter the same single container that you used when casting the spell, resulting in two souls being there. This would allow both you and your steed to attempt to possess nearby humanoids and take full advantage of the spell, even letting your non-humanoid steed possess humanoids. As far as I can tell, nothing actually goes wrong or acts weirdly from there being two souls at once and nothing explicitly states that only humanoids can enter the container.
Find (greater) steed might not work with magic jar due to targeting
The find steed spell states (emphasis mine):
[...] While mounted on your steed, you can make any spell you cast that targets only you also target your steed. [...]
The magic jar spell states:
[...] You can attempt to possess any humanoid within 100 feet of you that you can see (creatures warded by a protection from evil and good or magic circle spell can't be possessed). The target must make a Charisma saving throw. [...]
This is a rather small thing, but one could argue that the creature being affected by the spell is considered a target of the spell and thus magic jar does not actually target only you, or at least, it has the potential to target somebody else. Though the find greater steed spell doesn't actually care about whether a spell might target somebody else, only if it actually does target somebody else (An example of something that cares about whether a given spell might target more people is the Sorcerer's Twinned Spell Metamagic).
Your GM might rule that magic jar simply doesn't work with find (greater) steed because of targeting, this would be similar to the stance taken in this answer to a question about the interaction between find (greater) steed and the various smite spells. Note that what a given spell targets is not an agreed upon thing (we have a lot of questions on spell targeting).