I think it really depends on what exactly the litter is made of and whether it is biodegradable or not. Many electronics contain harmful metals, that if leaked into the soil could contaminate the water supply, and that harms everyone, including animals. Other litter is harmful in that it poses a danger to animals who attempt to consume it, or who become physically entangled in it, and die from suffocation. Even paper products can be printed with inks that may be poisonous when swallowed, or in any case they put chemicals into the environment that were not there originally, which can be potentially harmful if they get into the water table. That's just what I could think of off the top of my head.
