I recently had a thought about Oxyotl. I know that it is best to put him in a unit of Chameleons, but if you didn't, what would happen to his Chameleon rule? It says that enemy units shooting at a unit with this rule get -1 to hit, but what if Oxyotl is the only model in the unit with the rule? What then? Does the penalty simply not apply (unless he was being sniped)?
He does get the -1. He is a chameleon after all. Oxyotl is a great model. You should find that he is more of a psychological warfare model than anything else. Once your opponent knows he is in your list, it will be a threat that he has to deal with. I once played a High Elf player that altered the deployment of of his entire Army because he had Teclis and was worried that Oxyotl was in my list. Great model with good abilities.
Thing is that the enemy won't be directly targeting him just the unit, so how do you resolve the penalty?
As silly as it may seem, Oxyotl does not have the skirmish rule. Accordingly, he must follow all the rules for normal characters joining units. That being the case, if he joins a unit of chameleons, he gets the skirmish bonuses and is thus -1 to shoot at for skirmish and -1 since he has the chameleon rule. The unit is -2 to shoot at due to skirmish and chameleon. He can also march and shoot. Any shooting directed at the unit follows the normal rules (i.e. if 5+ R&F are present, all hits go against the unit, otherwise they are allocated by the owning player). If Oxyotl is on his own, he only gets the -1 bonus for chameleon when shot at and also loses the march and shoot ability since he does not have the skirmish rule.