i am wild

Animorphs told the story of five kids granted the power to become any animal they could touch by a dying alien who crash-landed in front of them on their way home one night. By accepting this power they became guerrilla soldiers in an invisible war: they were told by the alien (a technologically advanced, blue, centaur-like creature called an Andalite) that the Earth was and is under attack by another race of sentient aliens called Yeerks. Yeerks are parasites, able to invisibly control the creatures they infest. The Andalites pursue and attempt to thwart the Yeerks across the galaxy, but their help may or may not be coming to Earth...and in the meantime, the only force standing between the planet and total, silent subjugation is our heroes. 

For the purpose of this AU, Elfangor the Andalite happened to crash in front of a different group of Earthlings: a couple of familiar Amis. The rest is history.

The Morphing Power

The morphing power is bestowed by a piece of Andalite technology called an escafil device, which resembles a small, smooth-sided blue cube that can fit in the palm of one's hand. It is activated by touch. 

In order to morph an animal, its DNA must be 'acquired' by touch. DNA must be acquired from a living animal: a dead creature, or something like a sample of blood or fur, will not suffice. This also naturally excludes the possibility of acquiring DNA from the remains of extinct creatures. Plants, single-celled organisms, and inanimate objects cannot be morphed. A morph cannot be acquired from another person's morph, and you cannot acquire any new morph while already in a morph. 

Acquiring an animal requires about a minute of concentration, and places the animal in question into a sort of docile trance while the event transpires. 

The act of morphing itself requires a good amount of concentration as well, and the process of transformation is only rarely graceful or sensible. It can take about two minutes to morph completely, though this interval of time can be shortened with practice. With effort, some little amount of skintight clothing can be incorporated into a human morph, though bulky objects like coats, glasses, or shoes simply cannot be. 

When in morph, a morpher takes on not only the body of the animal in question, but also aspects of its instinct and mind. This is helpful, if not vital, to the process--what does a human mind know about how to echolocate, or fly?--but comes with its set of quirks and drawbacks. Overcoming the animal mind is challenging when becoming accustomed to a new morph, and acting contrary to the animal's instincts is always challenging.

A morpher cannot shift from one morph directly to another. They must instead 'demorph' to their natural state before choosing another shape. Morphing many times repeatedly can be tiring. 

When in a morph, morphers can communicate via thought-speech, the same language the mouthless Andalites use. It is <designated like this> as opposed to with quotations marks, and can function both like regular speech and as private telepathy--one can choose where and to whom they address it, and its volume. 

Finally, there is a strict time limit to staying in morph. If you stay in a morph for more than two hours, you become stuck in that morph. A morpher thus frozen can never return to their natural form, nor can they be given the morphing power again with the escafil device. 

Aliens

Humans are far from the only sentient race in the galaxy in this 'verse, and are in fact some of the more technologically simple and ignorant ones. The key aliens to know are those that are involved with the invasion of Earth:

Andalites are a technologically advanced race of aliens renowned across the galaxy for opposing the Yeerks. They resemble centaurs, with a humanoid torso and the lower body of a deer-like ungulate. One of their most striking features is the long tail ending in a sharp blade, which they use proficiently as a weapon and carry curved over their backs like scorpions. They have four eyes, two in the usual location and two mounted on stalks atop their head, which can look in any direction. They have no mouth or humanoid nose, the lower parts of their faces instead being occupied by a feature described like three vertical slits. They have dexterous hands with seven fingers on each. 

Yeerks are the slug-like aliens responsible for the impending takeover of Earth. In their natural state they are incapable of sight, most locomotion, and verbal communication; instead they are endowed with the ability to enter a host's brain through their ear canal and entirely take over their body. A Yeerk thus occupying a host is referred to as a 'Controller'. Yeerks subsist on what they call Kandrona, a nutrient naturally produced by the sun on their homeworld. Invading Yeerks must leave their hosts once every three days to swim in a Yeerk Pool and absorb artificially generated Kandrona; this is one of the invasion's points of vulnerability. 

Hork-Bajir are a race of aliens that have entirely fallen to the Yeerks; all Hork-Bajir are Controllers. They are bipedal and reptilian, with long snakelike necks and beaked faces. Growing up to seven feet in height, they are large, dangerous, and favored by the Yeerks as shock troops--in no small part due to another feature of their anatomy, the lethal blades that project from their wrists, elbows, knees, ankles, forehead, and tail. 

Taxxons are another race subjugated by the Yeerks, though unlike the Hork-Bajir they willingly became controllers. They resemble nothing more than giant centipedes that travel on many legs with the front third or so of their body carried off the ground, from which twelve lobster-clawed appendages project. Their head terminates in a round mouth full of teeth, surrounded by four red, gelatinous eyes. They're always hungry. 

So, that's all I can think of in terms of basic information for now (and to be honest it's probably already FAR TOO MUCH but, whatever, it's good to have reference all in one place). Once again, if you've got questions or want to get involved, just let me know!