The answer is dumping or abandonment of any animal in the highway. It naturally is a violation of the California Vehicle Code’s Section 21376, which states: “The Department of Transportation shall place and maintain on each major state highway entering the state, within 500 feet after the state line, a symbol that states the abandonment or dumping of any animal (on a highway) is a delinquency punishable by a fine of up to $1,000 or confinement in a county jail up to six months, or both.”
In some states, deserting an animal anyplace – not just on a highway – is illegal. In California, the Penal Code’s Section 597 mentions that it’s a crime for an owner of any animal to permit it to be without proper care and attention, which would seem to include intentionally abandoning it anywhere.