Encumbrance for characters is detailed in all RM editions. I can't find nowhere about beast of burden, horses, mules etc. The only insight is transport price chart, in character law different editions, who summarize carry capacity. Is the topic anywhere else discussed?
I presume horses speed could decrease every 10% of beast weight over the carrying capacity.
Example: light horse, weight 800 lb. carry 200 lb. - 10' base movement every 80 lb. carried above 200 lb.
Make it sense?