Where the shape comes from
An ox drawn head-on and reduced to what tells it apart: the two horns curving up from the crossbar, and the muzzle below. Compare 羊, where the horns curve down instead. Cattle were the great sacrificial and working animal, so the radical marks both the beast and what is done with it: 物 originally a coloured ox, 特, 牧, and the 牛奶 on any breakfast table.
汉字 filed under 牛
No published character on HSK Guide is filed under this radical yet. 汉字库.