I read in an answer to Druid (Sentinel) Animal Companion Armor that there was at least one item (Companion's Defender) which could be given to a Sentinel Druids Animal Companion, but as I am not a DDI subscriber, I couldn't follow the link for the list of other items.
I did find another item listed in Bunko's Bargain Basement 4E:
Companion's Defender: A solid scaling boost your your companion's defenses.
Guardian's Collar: Give your buddy a nice bonus to perception checks,and double the chance to not be surprised.
Alas, to cut down on clutter, Bunko's only lists items of at least a Black rating.
So, are there any other items available to for an Animal Companions item slot?