mycetic spores are what tyranids use to attack planets. they dont have any landing vehicles or anything, everything tyranid on the planet comes down in one of these. even bio-titans!
i searched forge world and they do not have a model for these, but IA4, which i dont have, might have rules for them.
here is a picture of roughly what they look like, though in my mind they arent nearly as fat, much longer, and would have little ganglia things on the back end. just for laughs
if they did have rules and such for these, you would probrobly use them much like a drop pod, as you said. probrobly drop a slow moving, but heavy hitting, unit close to enemy lines. like a carnifex.