It STARTS size large? Huh...
Well, being large it has 4x normal carrying capacity, and you can buff the str to 16 with the point at 4 HD (but I'd do Int first to open up any feat you want for it to take). If it doesn't need the str to carry you, I would buff int, then dex, then str for its 4th, 8th, and 12th HD.
Basic setup for archer ranger otherwise. Consider Mounted Combat and Trick Riding to help protect it. You could do mounted combat style to get Trick Riding early, but I don't think that's worth giving up IPS at 6 and Point Blank Master at 10.
I would always try to get to ranger 6 ASAP, but you'll need boon companion anyway, so that means you could dip a level somewhere else and still not lose companion advancement. Emissary Cavalier 1 will get you free Mounted Combat. Sohei Monk lets you pick up any mounted combat feat w/o pre-reqs, so you could actually get Trick Riding at very low levels with a dip there.