Just an FYI. Great mod btw!

[24Jan2021 09:09:37.105] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for grizzly_bear
[24Jan2021 09:09:37.114] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for roadrunner
[24Jan2021 09:09:37.129] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for bone_serpent
[24Jan2021 09:09:37.131] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for bone_serpent_part
[24Jan2021 09:09:37.138] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for gazelle
[24Jan2021 09:09:37.165] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for crocodile
[24Jan2021 09:09:37.171] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for fly
[24Jan2021 09:09:37.176] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for hummingbird
[24Jan2021 09:09:37.183] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for orca
[24Jan2021 09:09:37.189] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for sunbird
[24Jan2021 09:09:37.210] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for gorilla
[24Jan2021 09:09:37.220] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for crimson_mosquito
[24Jan2021 09:09:37.222] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for mosquito_spit
[24Jan2021 09:09:37.224] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for rattlesnake
[24Jan2021 09:09:37.242] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for endergrade
[24Jan2021 09:09:37.245] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for hammerhead_shark
[24Jan2021 09:09:37.247] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for shark_tooth_arrow
[24Jan2021 09:09:37.254] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for lobster
[24Jan2021 09:09:37.262] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for komodo_dragon
[24Jan2021 09:09:37.292] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for capuchin_monkey
[24Jan2021 09:09:37.294] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for tossed_item
[24Jan2021 09:09:37.298] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for centipede_head
[24Jan2021 09:09:37.299] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for centipede_body
[24Jan2021 09:09:37.311] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for centipede_tail
[24Jan2021 09:09:37.325] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for crocodile_egg
[24Jan2021 09:09:37.339] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for warped_toad
[24Jan2021 09:09:37.346] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for moose
[24Jan2021 09:09:37.351] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for mimicube
[24Jan2021 09:09:37.360] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for raccoon
[24Jan2021 09:09:37.366] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for blobfish
[24Jan2021 09:09:37.381] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for seal
[24Jan2021 09:09:37.426] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for cockroach
[24Jan2021 09:09:37.432] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for cockroach_egg
[24Jan2021 09:09:37.468] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for shoebill
[24Jan2021 09:09:37.519] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for elephant
[24Jan2021 09:09:37.540] [modloading-worker-12/ERROR] [net.minecraft.util.Util/]: No data fixer registered for soul_vulture

With:
alexsmobs-1.4.1.jar
1.16.4-forge-35.1.28

Data fixers are used to update data from old worlds to new formats. Since we don't normally keep modded worlds between versions, this is not a big deal.