Those three mules of the three new rhino riders
Now that three of the four people that want to ride rhinos have them, we've three mules to consider possibilities.
Given the situation, I'd consider each of those mules to be the "property" of their ex-rider, so any decision would be up to them, but there might be something of value that the Rhino clan has that someone with an "extra" mule could trade to them. That might be tack/harness or lance, etc., for use with the rhinos, if that wasn't given to you, or durables/gear that the party might need to survive on the plains.
Of course keeping the mules is always an option- that's future food riding along with us if it would become a need.