Saving Private Rudolf

Let's see: as I understand it (and I think I understand it) while deer is the plural inflection of deer, a killdeer is not actually a deer so it should be necessary to add an S in order to make it plural. Although, according to Merriam Webster, killdeer and killdeers are both acceptable, but they also accept portmanteaus like ginormous as legitimate english, so what do I care what they think?