The only one that comes to mind for me is
Lewisite, which was first synthesized before the war but not used in combat. It's like mustard gas, but somewhat more dangerous. It's not nearly as deadly as the nerve agents, but no nerve agents were developed until the 1930s and I don't think 1910s chemistry was up to the task of making nerve agents.
Although not formally a chemical weapon,
PCBs cause significant environmental damage and some PCBs were known in 1914-1918. The environmental impact wasn't known then, but it is conceivable that it could have been. PCBs wouldn't be a true chemical weapon capable of killing or seriously injuring soldiers, but they could be deliberately released at the front in a scorched-earth context, to damage the ecosystems there for a very long time. Compounds of mercury or arsenic compounds such as
Paris Green could also have been deliberately spread in large amounts to cause environmental damage.