Alien Ships (Apocalypse)

In 1997 game X-Com: Apocalypse, last remaining human city, Mega-Primus, is assaulted by aliens from another dimension. Aliens appear from dimensional portals with various ships or UFO's that are grown instead of being built.

Overview
Most of the alien technology consists of organic matter, and their flying saucers are no exception, though it's debatable if the bioconstructs actually fly. Alien technology allows them to warp space next to the saucer to match that of alien dimension, which constantly repositions the craft to new position.

Ships are grown in their home dimension in a specific building-organ, and emerge from a stalk that severs the mushroom cap that then acts as an organic UFO. It is suggested that the ships are powered by machinery that can suck energy from alien dimension which transforms into antimatter in our dimension. And as aliens have learned to harness antimatter, they have tremendous amount of power at their disposal. Apparently their propulsion system takes a major toll in their energy supply, though.

Alien weapon and control systems show some mismatch in otherwise entirely organic technology. This could be because of some outside influence.

Role in invasion
Alien creatures need a method to travel through dimensional portals, as entering one without proper protectiong results in complete annihilation. Alien craft are large enough to house such technology.

Typically alien ships are crewed by Anthropods and Skeletoids, with an assortment of other alien creatures depending on their mission. Early in the invasion ships often drop Multiworms and Hyperworms  along with their nannies, Spitters, to create a base of operations and to have a continuous presence in our dimension. Later, when war escalates, creatures that are built solely for warfare, such as Poppers and Megaspawns, start to appear.

Ships in alien fleet have different purposes, ranging from probing our dimension to full fledged assault aimed at dropping a dreaded Overspawn to Mega-Primus. Latter is often because aliens have given up on trying to infiltrate various organisations, and instead want to beat humans or at least cause as much damage as possible.

Different examples of alien ship design in gallery below.