You can get semimega/megabeasts playable in genesis this way.
Make a .txt file called "entity_monster.txt" in your /raw/objects folder of the Genesis mod installation.
Put the following text in it:
entity_monster
[OBJECT:ENTITY]
=====================================================================================================================================
Adventure tiers:
1 - dwarves
2 - sylvan elves
3 - dusk elves
4 - ezrakim elves
5 - keepers
6 - nords
7 - orcs
8 - goblins
9 - serpentfolk
10 - kobolds
11 - dark dwarves
12 - illithids
13 - werewolves
14 - centaurs
15 - scavengers
16 - demons
17 - hydra
18 - red dragon
19 - blue dragon
20 - wyvern
21 - balor
22 - deep crow
23 - cave dragon
24 - roc
25 - bronze colossus
26 - giant
27 - cyclops
28 - ettin
29 - minotaur
30 - thunderbird
=====================================================================================================================================
[ENTITY:ENTITY_HYDRA]
[ADVENTURE_TIER:17]
[INDIV_CONTROLLABLE]
[CREATURE:HYDRA]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_DRAGON]
[ADVENTURE_TIER:18]
[INDIV_CONTROLLABLE]
[CREATURE:DRAGON]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_DRAGON_BLUE]
[ADVENTURE_TIER:19]
[INDIV_CONTROLLABLE]
[CREATURE:DRAGON_BLUE]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_WYVERN]
[ADVENTURE_TIER:20]
[INDIV_CONTROLLABLE]
[CREATURE:DRAGON_GREEN]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_BALOR]
[ADVENTURE_TIER:21]
[INDIV_CONTROLLABLE]
[CREATURE:BALOR_DETAILED]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_CROW_DEEP]
[ADVENTURE_TIER:22]
[INDIV_CONTROLLABLE]
[CREATURE:BIRD_CROW_DEEP]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_DRAGON_CAVE]
[ADVENTURE_TIER:23]
[INDIV_CONTROLLABLE]
[CREATURE:CAVE_DRAGON]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_ROC]
[ADVENTURE_TIER:24]
[INDIV_CONTROLLABLE]
[CREATURE:BIRD_ROC]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_COLOSSUS_BRONZE]
[ADVENTURE_TIER:25]
[INDIV_CONTROLLABLE]
[CREATURE:COLOSSUS_BRONZE]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_GIANT]
[ADVENTURE_TIER:26]
[INDIV_CONTROLLABLE]
[CREATURE:GIANT]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_CYCLOPS]
[ADVENTURE_TIER:27]
[INDIV_CONTROLLABLE]
[CREATURE:CYCLOPS]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_ETTIN]
[ADVENTURE_TIER:28]
[INDIV_CONTROLLABLE]
[CREATURE:ETTIN]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_MINOTAUR]
[ADVENTURE_TIER:29]
[INDIV_CONTROLLABLE]
[CREATURE:MINOTAUR]
[TRANSLATION:LATIN]
[ENTITY:ENTITY_THUNDERBIRD]
[ADVENTURE_TIER:30]
[INDIV_CONTROLLABLE]
[CREATURE:BIRD_THUNDERBIRD]
[TRANSLATION:LATIN]
You can add more creatures to the list, just copy-paste the
[ENTITY:ENTITY_THUNDERBIRD]
[ADVENTURE_TIER:30]
[INDIV_CONTROLLABLE]
[CREATURE:BIRD_THUNDERBIRD]
[TRANSLATION:LATIN]
Replace ENTITY_THUNDERBIRD with an
unique entity name.
Increment the adventure tier so it's more than the previous ones.
Replace BIRD_THUNDERBIRD with the ID name of the creature you want.
Don't forget that you have to make a new world for this to work.