The following macro does not remove the “Magmadar’s Return” buff:
#showtooltip
/cancelaura Magmadar’s Return
/use Swift Brown Steed
I have checked the name of the buff and it matches here: https://www.wowhead.com/classic/spell=461270/magmadars-return
I can right click it, but when I’m farming and want to mount I just want to click my mount button, not find and right click the buff, then mount.