I don't think I have a merman.
Here are the mobs available:
STANDARD:
-creeper
-skeleton
-spider
-zombie
-slime
-ghast
-zombie pigman
-enderman
-cave spider
-silverfish (no, it'd kill the castle blocks)
-blaze
-magma cube
-witch
ATUM:Journey into the Sands
-mummy
-brigand
-nomad
-pharaoh
-forsaken
-wraith
-tombguard
-desert wolf
-warlord
EREBUS:Dimension of Arthropods
-maggot
-beetle
-fly
-greenfly
-mosquito
-tarantula
GRIMOIRE OF GAIA 2
-anubis
-banshee
-baphomet
-bone knight
-centaur
-chochin-obake
-cobblestone golem
-cockatrice
-creep
-dryad
-dullahan
-ender dragon girl
-ender eye
-flesh lich
-cyan flower
-ghost
-giga knight
-gnome
-gryphon
-harpy
-hunter
-mandragora
-mermaid
-mimic
-minotaur
-minotaurus (different)
-naga
-sahuagin
-shaman
-siren
-slime girl
-sphinx
-spider queen
-spriggan
-stone golem
-succubus
-valkyrie
-vampire
-werecat
-witch (different)
-wither cow
-yeti
-yuki-onna
CHOCOCRAFT (friendly)
-Chicobo (baby chocobo)
-Chocobo (yellow, green, blue, white, black, gold, pink, red, purple)
MO'CREATURES (listing only hostiles)
-HorseMob
-Ogre
-Boar
-bear
-bigcat (tiger, lion, panther)
-wwolf (white wolf)
-wraith (different)
-flamewraith
-werewolf
-shark
-rat
-hellrat
-scorpion
-crocodile
-big golem
-komodo dragon
-wyvern
-minigolem
-silverskeleton
-piranha
LEGENDARY BEASTS
-fire demon
-ender lord
-lightning spirit
-elf hunter
-snow beast
LAVA MONSTERS
-lava monster
MUTANT CREATURES
-mutant creeper
-creeper minion
-mutant zombie
-mutant enderman
-mutant snow golem
MYTHS & MONSTERS
-sand wyrm
-golden ram
-fairy
-wolpertinger
-jackalope
-frost jackalope
-dendroid
-dendroid elder
-void ursa
-celestial ursa
-hunter
-elite hunter
-hill giant
-kraken
-cetus
PRIMITIVE MOBS
-haunted tool (poltergeist)
-treasure slime
-rocket creeper
-festive creeper
-brain slime
-support creeper
-mother spider
-mimic chest (different)
-skeleton warrior
-summoner
-bewitched tome
REPTILES
-way too many to mention, but expect alligators, more turtles, savanna monitors, etc.
TWILIGHT FOREST
-wild boar
-bighorn sheep
-wild deer
-redcap
-swarm spider
-naga (different, boss)
-skeleton druid
-hostile wolf
-twilight wraith
-hedge spider
-hydra (boss)
-twilight lich (boss)
-kobold
-mosquito swarm
-death tome
-minotaur (yes, third kind)
-minoshroom
-fire beetle
-pinch beetle
-slime beetle
-maze slime
-redcap sapper
-mist wolf
-king spider
-carminite ghastling
-carminite ghastguard
-carminite golem
-towerwood borer
-carminite broodling
-Ur-ghast (boss)
NON-SPAWNEGG (there's no way to get these show up but with random chance, as they 'replace' random vanilla mobs)
SPECIAL MOBS
-Fire, Ice, Lightning Creepers
-I think four or so spiders
-gatling skeleton, thief skeleton, dash skeleton, etc.
...basically extra versions of vanilla mobs