My Minercaft Server warns me about this and I don't really know if this is an big issue because actually everything works finde. At least I didn't notice anything that could be wrong.
Thanks for handeling this "problem". :D

[09:27:43] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityGoat from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:43] [Server thread/INFO] [minecraft/LoggingChunkStatusListener]: Preparing spawn area: 10%
[09:27:43] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityBearNeutral from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:43] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityBoar from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:43] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntitySquirrel from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:43] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityPheasant from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:44] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityDeer from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:44] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntitySongbird from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:44] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityCoyote from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:27:44] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityLammergeier from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:27] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityReindeer from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:27] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityButterfly from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:27] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityButterfly from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainerContainable
[09:30:28] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityFreshwaterEel from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:28] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityFreshwaterEel from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainerContainable
[09:30:28] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityBadger from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:29] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityDragonfly from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:29] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityDragonfly from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainerContainable
[09:30:30] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityLamprey from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:30:30] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityLamprey from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainerContainable
[09:30:33] [Server thread/WARN] [minecraft/MinecraftServer]: Can't keep up! Is the server overloaded? Running 2035ms or 40 ticks behind
[09:30:42] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityTurkey from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:44:28] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityFeralWolf from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer
[09:44:34] [Server thread/WARN] [minecraft/EntityDataManager]: defineId called for: class its_meow.betteranimalsplus.common.entity.EntityGoose from class dev.itsmeow.betteranimalsplus.imdlib.entity.util.EntityTypeContainer

@LotP1 There's nothing I can do to suppress it otherwise I would. I agree with you. It does only show once per entity per world startup, though.

the console spam is really annoying tho, and makes it hard to keep track of important warns in the server console.

This is a useless warning and means nothing. No worries!