The Tumor Familiar discovery from the Alchemist base class states:
Benefit: The alchemist creates a Diminutive or Tiny tumor on his body, usually on his back or stomach. As a standard action, the alchemist can have the tumor detach itself from his body as a separate creature vaguely resembling a kind of animal suitable for a familiar (bat, cat, and so on) and move about as if it were an independent creature. The tumor can reattach itself to the alchemist as a standard action. The tumor has all the abilities of the animal it resembles (for example, a batlike tumor can fly) and familiar abilities based on the alchemist’s caster level (though some familiar abilities may be useless to an alchemist). The tumor acts as the alchemist’s familiar whether attached or separated (providing a skill bonus, the Alertness feat, and so on). When attached to the alchemist, the tumor has fast healing 5. An alchemist’s extracts and mutagens are considered spells for the purposes of familiar abilities like share spells and deliver touch spells. If a tumor familiar is lost or dies, it can be replaced 1 week later through a specialized procedure that costs 200 gp per alchemist level. The ritual takes 8 hours to complete.
What is unclear here is, does the Alchemist choose the form his tumor resemble when he makes the discovery, or does he choose a shape every time the tumor detaches itself? The wording would suggest that the creature is chosen every time the tumor detach itself, but then it states that the tumor also acts as a familiar when attached to the alchemist, giving it the relevant bonuses. This suggests to me that only one shape might be chosen and that is the shape all the time, every time.