The original blimp design. This blimp was large, round, heavy, had little lift and would explode if you took it too high. Not very useful for interplanetary fun in KSP, but it did a good job of approximating the effect of buoyancy.
The original modder had Ludo in their screenname, little else is known after so much time has passed. Hooligan Labs picked up the mod and added the additional envelope type, spaceworthyness and all the neat controls.
The first and least interestingly named envelope by Hooligan Labs.
Eight times the size of the Envelope, this is commonly used as it is not too large while having substantically more lift. Naturally, it is named after the eight arms of an octopus.
The most prominent inspiration for the Airships to Other Planets was JP Aerospace. They have proposed a three stage airship system to slowly but efficiently put stuff into orbit. The first stage ascends from earth to a floating high altitude base. There, the cargo is transferred to an even larger airship which accelerates to orbital velocity while on the fringes of the atmosphere using ion engines.
All attempts to recreate this in KSP have failed, by the way.