MobArena supports any monster available in the EntityType enum. You don't have to write the names in all caps, and you can omit or include underscores (_), hyphens (-), and periods (.) as you please.
Some monsters are a little special. Creepers, for instance, can be charged or powered, meaning their explosions become much more powerful, and their appearance changes. Another example is slimes and magma cubes, which have different sizes.
MobArena supports some of the variations of these different monster types, and they are listed here:
angrybeeis an aggressive beeangrywolfis an aggressive wolf with red eyesbabydrownedis a baby-version of a drownedbabyhoglinis a baby-version of a hoglinbabyhuskis a baby-version of a huskbabypiglinis a baby-version of a piglinbabypigmanis a baby-version of a piglin (legacy)babyzoglinis a baby-version of a zoglinbabyzombieis a baby-version of a zombiebabyzombievillageris a baby-version of a zombie villagerexplodingsheepis a MobArena-specific type of sheep that bounces around and explodes when in the proximity of a playerkillerbunnyis a killer bunny version of a rabbitpoweredcreeperis a charged/powered creeper
As for slimes and magma cubes, both monster types are assigned a random size when they spawn. However, they also both support size suffixes that force them to be a specific size. They are:
tinysize 1smallsize 2bigsize 3hugesize 4
As an example, slimehuge will spawn a size 4 slime, while magmacubetiny will spawn a size 1 magma cube.