The baku are composite creatures from Japanese mythology: sometimes they sport the head of an elephant and the body of a lion. As you know, the baku in Betrayers of Kamigawa (Blademane Baku, Petalmane Baku, Quillmane Baku, Skullmane Baku, Waxmane Baku) are a cycle of spirits that gain abilities as you use Arcane magic.
But what is a "mikoshi," or a "palanquin"?
A mikoshi is a portable Shinto shrine that serves as the vehicle of a divine spirit in Japan at the time of a parade of deities. Often, the mikoshi resembles a miniature building, with pillars, walls, a roof, a veranda and a railing. Typical shapes are rectangles, hexagons, and octagons. The body, which stands on two poles (for carrying), is usually lavishly decorated, and the roof might hold a carving of a phoenix.
A palanquin is a covered conveyance, usually for one person consisting of a large box with wooden shutters like Venetian blinds, carried by four or six (rarely two) men by means of poles projecting before and behind.
So intead of having a miniature building or wooden box, Baku Altar has a statue of a baku. Now you know.