Arena is finicky with that..... a lot of the game is finicky.
To get a unique creature, it has to be named uniquely in the code.
[creature:xyz]
[name:xyzwolf]
is the same creature as
[creature:xyz]
[name:xyz]
The [creature:____] is the part telling the game that this is the unique id of the critter, and that all the stuff below, until the beginning of the next [creature:____] tag is part of it.
in addition to making a unique id for the monster, you have to give it a creature name. [name:xyz]. in arena, creatures get further defined into caste names.
[CREATURE:DIONAEA]
[DESCRIPTION:A sentient and mobile plant with a robust digestive system.]
[NAME:dionaea:dionaeii:dionaea]
[CASTE_NAME:dionaea:dionaeii:dionaea]
[GENERAL_CHILD_NAME:immature dionaea:immature dionaeii]
[CREATURE_TILE:'D'][COLOR:6:0:0]
This makes sense when it is explained, since arena exists to test out monsters and some monsters have gender differences or can give birth to a 'special great child'. One of my monsters has a male population of 98%, and another has a 1/1000 chance to birth a special male who is bigger, stronger, dodgier, and a smooth talker. I wouldnt be able to control these specific castes if the game didnt let me specify what caste I wanted to spawn in the arena.
Think of castes as gender identities if it helps, or as ethnicity. The caste system in DF can simulate both if you wanted it too.
Give them a caste name, a creature name, and a unique [creature:___] name. Then, check the
error log to make sure something bigger is not to blame for your creature not appearing.