In Latin, prædātōrius:prædātor::prædātus:prædātīcius/prædātītius.
Words prædātor and prædātus are antonyms.
Corresponding adjectives are prædātōrius and prædātīcius.
Therefore English antonym to predatory should be predaticious/predatitious (Latin meaning “taken as booty or plunder” per Lewis & Short).
Suffix -itious/-icious means “having the nature of”. From this, we can confirm the meaning of predaticious/predatitious as “having the character of the predated (preyed upon)”.
(Sources: Dictionary.com, The Free Dictionary, Merriam-Webster.)
An excited predator might exclaim: “Mhmhm... This prey looks very predaticious!”
However, the only two authors to ever use this word in English (bishop John Gauden and gardener John Evelyn) have used it incorrectly as a synonym (rather than antonym) of the word predatory. You have an opportunity to fix their historical mistake.
There is also an English word predacious/predaceous. Again, it is used as a synonym (rather than antonym) of the word predatory. Again, the correctness of this usage can be questioned: Latin word prædātiōsus/prædāciōsus is not attested. If it existed, it would mean something to the effect of “plundersome” – hard to say whether a synonym or antonym. Whereas Latin word prædātius/prædācius means “more predated” (comparative degree of praedātus).
For a more common word, I would settle for submissive or gullible. How about furtive? A prey aware of being a target may behave furtively. Furtive behaviour defines a prey.
- John Gauden, Bp., “A sermon preached (...) at the funeral of (...) dr. Brounrig” (1660, London):
Not predaticious to any, but propitious to all true Saints.
- John Evelyn, “A Discourse of Forest-trees and the Propagation of Timber” (1679 Third Edition, possibly also 1662 paper):
It is not good to water new-ſown Seeds immediately, (...) be ſure to
purge them of predaticious Weeds betimes.
Provided no rank Weeds, or predatitious Plants (conſummating their
Seeds) be ſuffered to grow and exhauſt it.
Also compare this passage:
But the ſhade of the Aſh is not to be endur’d, becauſe it produces a
noxious Inſect; and for diſplaying themſelves ſo very late, and falling
very early, not to be planted for Umbrage or Ornament; eſpecially near
the Garden, ſince (beſides their predatitious Roots) the deciduous leaves
dropping with ſo long a Stalk, are drawn by cluſters into the Worm
holes, which foul the Allies with their falling Keys, and ſuddenly
infect the ground.
with corresponding passage in John Mortimer’s anthology “The Whole Art of Husbandry” (1721):
But the ſhade of the Aſh is not to be endured, becauſe it produces a
noxious Inſect; and becauſe of the late Budding, and early falling of
the Leaves, and therefore ’tis not to be planted for Walks or Ornaments,
eſpecially near Gardens, because of their ſpreading Roots and falling
Leaves, both which are prejudicial to them.
This change clearly shows the intended meaning of “predatitious” as “predatory” (expansive, “spreading”) and possibly acknowledges it as an error.
- John Evelyn, “Elysium Britannicum, Or the Royal Gardens” (unpublished manuscript until 1998 edition by Therese O'Malley, Joachim Wolschke-Bulmahn, and 2001 edition by John E. Ingram):
And yet I remember Columella (...) is not for a total extermination
even of those laizy & predatitious Bees, (...)
least the labourious Bees should grow idle.
- Oxford English Dictionary (article behind paywall):
- Aulus Gellius, “Attic Nights” (book 13, chapter XXV, section 28):
Itaque hæc inscriptio quam videtis: ‘Ex manubiis,’ non res corporaque
ipsa prædæ demonstrat, nihil enim captum est horum a Traiano ex
hostibus, sed facta esse hæc conparataque ‘ex manubiis,’ id est ex
pecunia prædaticia, declarat.
Therefore this inscription which you see: ‘From the spoils,’ does not
demonstrate the things and the matter of the prey (loot) itself.
For none of these were taken from the enemy by Trajan. It only declares
that they were made and bought ‘from the spoils,’
i.e. from predaticious money.