Some of this is covered in the video tutorial Modding with an Iron Fist: Episode 3: Adding New Creatures: http://www.youtube.com/watch?v=jLEjN6YkvEQ

Attributes of the "creature" tag

id

A unique ID for the creature. IDs should be in ascending order; i.e.: if the last creature has ID 77, your new creature should have ID 78.

name-singular

 The singular form of the creature's name. Should be lowercase. Example: "harpy"

name-plural

The plural form of the creature's name. Should be lowercase. Example: "harpies"

icn

The name of the .ICN sprite archive file which contains the creature's battle graphics. See Adding Artwork for instructions on how to create these files and get them in the game. Case-insensitive. Example: "harpy.icn"

frm

The name of the FRM.BIN creature-frame information file which tells the game how the creature's art fits into animations. See Controlling Creature Animation for instructions on how to create these files and get them in the game. Case-insensitive. Example: "hrpy_frm.bin"

cost

The cost to recruit the creature in gold. At time of writing, you cannot assign a creature's secondary resource costs. (Please ask if you want this; it should not be too hard.) Example: "500"

fight-value

A number which tells the game AI how "good" the creature is. Should range between 0 and 65535. Example: "28144" (for the Black Dragon, the highest fight-value in the game).

 <creature id="77" name-singular="treant" name-plural="treants" icn="treant.icn" frm="dend_frm.bin" cost="500" fight-value="1690" fight-value-aux="34" growth="3" hp="120" faction="6" speed="2" attack="6" defense="18" shots="0" short-name="dend">

         <damage minimum="3" maximum="7"/>

         <random-spawn minimum="10" maximum="20"/>

     </creature>