Possible Clues:

• Silver in old Westerns
• Amount of the start of 35 across
• 12 across this to get more than half of 26 down
• "Giddyup!" obeyer
• Smack the pony
• Stallion or mare
• Cavalry out of Spain
• Word with saw or sea
• "Three Men on a ___"
• Certain steed