Pressure (Notevember 2021, #9)

Deep down in the dark abyss,
one finds the felis tenebris.
A polymorphic group of creatures
who all share some common features:

• Black, and roughly feliform
  (though deviation from this norm
  is “not unheard of”, an aside:
  it hasn’t yet been verified)
• Adaptability to pressure;
  some can swim the depths at leisure;
  others, buoyant as a bubble,
  reach the surface without trouble
• Both aquatic and terrestrial
• Playful but, when threatened, bestial
• Greatly variable in size
• Whitish, luminescent eyes
• Active mostly during night
• Sometimes (?) capable of flight
• Sentient, and quite creative
  (range from mad to cogitative)

All else unknown at time of writing.
If you don’t count marks from biting,
my attempts have yielded naught –
for a shadowcat cannot be caught!