Zinc chloride is the common name for the compound ZnCl2. The reason it is known as zinc chloride rather than zinc II chloride lies in how we name chemical compounds.
In the naming of ionic compounds, the oxidation state of the metal can be indicated in a few different ways. The Roman numeral system is used for transition metals that can form more than one stable oxidation state. However, zinc is somewhat unique because it typically exhibits a single oxidation state of +2 in its compounds.
Because zinc only has one common oxidation state, it is often unnecessary to indicate that state in the name. This is why ZnCl2 is simply called zinc chloride instead of zinc II chloride. When naming zinc chloride, the ‘II’ is understood as it is the only form in which zinc commonly exists in ionic compounds.
So, in summary, ZnCl2 is referred to as zinc chloride because zinc only has a +2 oxidation state in its compounds, making the use of Roman numerals redundant in this case.