Alternate Modes

An Alternate Modes; commonly shortened to "altmode," "alt mode," or "alt-mode," also known as a "trans-form", a "transform", a "changeform", or an "alt-form", is a term used to describe a Transformers' non-robot mode, whether it be vehicle, animal, or otherwise. Various, more specific, terms are also sometimes used—vehicle mode, jet mode, tank mode, etc.

Some Transformers don't have a humanoid form, so in those special cases, the alt-mode is perceived to be the mode which is more geared towards utility or disguise while their robot mode is the one geared more towards interaction with other Transformers. For example, for the G1 characters Laserbeak and Ravage, their animal forms are their primary mode, and the disks their alternate modes. Even Grand Slam and Raindance, who transform from cassettes to vehicles, would be thought of as having a disk/cassette alternate mode, as their vehicle forms—while not humanoid or animalistic—are at least self-mobile.

Transformers with more than two modes are usually considered to have multiple alt-mode. (Punch/Counterpunch on the other hand has one alt-mode and two robot modes).

Transformers use these modes as disguises in order to conceal themselves from the human population. Most of the popular vehicle modes are cars, trucks, jets, tanks, etc. The part that allows them to transform into their alt modes is the T-Cog.

Known Alt Modes

 * Vehicle Modes
 * Beast Modes

Point Statement
When a Transformer takes on an alternate mode for the purpose of disguise, they are mimicking that form, they don't actually become what that form represents. They are "robots in disguise". To accomplish this, they physically alter their forms, and when necessary, use holograms to help maintain the disguise illusion, especially when battle damage or lack of a driver might otherwise give them away.

History
Back on Cybertron during the Great War, all Autobots and Decepticons, including Neutrals have heavily armed and armored Cybertronian vehicle modes.

When they first arrived on Earth, they seem to have to scan Earth vehicles they could find for a new mode, usually from others or whatever vehicles were nearby. Certain Cybertronians, like Shockwave, retains their Cybertronian vehicle modes for unknown reasons.

Synopsis
Airachnid, unlike others or somewhat Triple Changers, has four modes: Robot Mode, Vehicle Mode, Beast Mode, and Drill Mode.