The Musketeer is some part of royalty
No wonder the Three Musketeers are in Royal Arena...
Socially, Ram Riders are elites
We can make this assumption about most Legendary cards, but irregardless: someone would only take a ram as her battle steed if it didn’t mean much, economically at least, to her if she lost it. A sheep would probably have lots of value for its wool and young, and here the Ram Rider is, clearly not concerned about going broke if her ram is felled (well, to be fair they’d probably have more things to worry about when that happens). They probably have access to other sheep as well. Heck, even then she clearly still aren’t so bold as to take an ewe into battle - that has even more potential income to be lost, I guess.