"Young" Template
Most creatures do not spring, fully-formed, in an adult shape from the day of their birth; most go through an aging process, meaning immature versions of the creature may be encountered. Often, young creatures are encountered with their parent(s), and more often than not present little to no threat.
Transformation. * The young goes down one size (Medium to Small, Small to Tiny, etc)
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The young has below average (2-3 per Hit Die) hit points and goes down one hit die size (d10 to d8, d8 to d6, etc)
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Speed goes down by 3/4; at DM's discretion, may not have certain movement types yet (wings not grown, etc)
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Subtract 4 once, and subtract 2 twice, from Strength, Dexterity, and/or Constitution, without choosing an ability score twice.
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Subtract 4 once, and subtract 2 once, from Intelligence, Wisdom, and/or Charisma, without choosing an ability score twice.
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Most traits will not have emerged yet, and most actions will not be available until the creature is older.
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Attack actions go down half the number of damage dice, and one size die (so 2d6 becomes 1d4, 10d10 becomes 5d8)