The following names can be used interchangeably: Xenobionta, Xenobionta user, Lonely Walker
1- Each Lonely Walker carry genes from all the animals in a group, ranging from a family( f.e. Felidae, the cats) to a phylum (f.e Mollusca, the moluscs).
2- Those animals can't be extinct (because the virus can only collect DNA from living animals). However, if an extant animal is a direct relative to an extinct group, the Xenobionta bearer can try to replicate their characters.
3- The animals can be from other imaginary populated planets, but the host must be a native intelligent species.
4- The maximum size allowed in a transformation is the one of the biggest species included. If the animals are smaller than a human the maximum is his base size.
5- The matter excess in transformation comes from a reverse energy-matter conversion powered by unknown cosmic radiations.
6- The Xenobionta with more simple development types have more plasticity than the most complex. For example, invertebrate-based ones usually have a base form that they modify on the fly, while vertebrate ones have from one to a few fixed forms.
The human chooses the traits he want to incorporate into the form and try to "unearth" them from their DNA.
7- Animal psychological traits mix with those of the human. If they are not kept at bay with sustained concentration they end replacing them and the transformation goes out of control.
8- Suffering near-fatal wounds will automatically revert the transformation. The tissue reconfiguration and high activity of stem cells can regenerate the damage and save the users' life, but following transformations will have a delay. A wound grievous enough to kill outright won't allow the regeneration to trigger.