I was just browsing through a mod for the Elemental Colony, and in that file, there were creatures ranging from CREATURE_THUNDERBIRD and CREATURE_181 to CREATURE_999
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meaning, in the Creatures.xdb file.
Yes, the unique creature ID can be either CREATURE_987 or CREATURE_TEST... in my opinion, for convenience, it's better to use CREATURE_TEST rather than CREATURE_987.
Sincerely, Psatkha