This is one of those obscure parts of building that certainly baffled me when I first ran into it!
The strange mob behaviour exhibited here is actually not a result of anything under the #MOBILES section, but the lesser-used #SPECIALS section towards the bottom of the area file. It's here that you can assign pre-made behaviours that go beyond the remit of any flag, though it's not always appropriate to use them. Have a look
here for more info.
In the case of the yyllethyn horse for example, assuming the horse's vnum for the area is 'QQ00', the file's specials section will read:
#SPECIALS
M QQ00 spec_pet_gen
S
I hope this helps clear it up.
"This is General Lath'lain Dy'nesir, of the Ebon Spur. Walking Murder surrounded by a thin veneer of civility."
-Miriel