As the title say, and I think it may be due to this mod, I think so because in the error there's a patch from big and small, but I'm unsure if it also patches animals, making the bigger ones pack animals, megasloth specifically seems to generate this error as listed:

Cannot generate arriving trader caravan for Ogre Tribes because there is a pawn kind (Megasloth) who is not a carrier but is in a carriers list.
UnityEngine.StackTraceUtility:ExtractStackTrace ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Log.Error_Patch5 (string)
RimWorld.PawnGroupKindWorker_Trader:GeneratePawns (RimWorld.PawnGroupMakerParms,RimWorld.PawnGroupMaker,System.Collections.Generic.List`1<Verse.Pawn>,bool)
RimWorld.PawnGroupKindWorker:GeneratePawns (RimWorld.PawnGroupMakerParms,RimWorld.PawnGroupMaker,bool)
RimWorld.PawnGroupMaker:GeneratePawns (RimWorld.PawnGroupMakerParms,bool)
RimWorld.PawnGroupMakerUtility/<GeneratePawns>d__2:MoveNext ()
System.Collections.Generic.List`1<Verse.Pawn>:.ctor (System.Collections.Generic.IEnumerable`1<Verse.Pawn>)
System.Linq.Enumerable:ToList<Verse.Pawn> (System.Collections.Generic.IEnumerable`1<Verse.Pawn>)

BigAndSmall.GeneratePawns_Patch:GeneratePawnsPatch

(RimWorld.PawnGroupMakerParms,bool,System.Collections.Generic.IEnumerable`1<Verse.Pawn>&)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:RimWorld.PawnGroupMakerUtility.GeneratePawns_Patch2 (RimWorld.PawnGroupMakerParms,bool)
RimWorld.IncidentWorker_NeutralGroup:SpawnPawns (RimWorld.IncidentParms)
RimWorld.IncidentWorker_TraderCaravanArrival:TryExecuteWorker (RimWorld.IncidentParms)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:RimWorld.IncidentWorker.TryExecute_Patch2 (RimWorld.IncidentWorker,RimWorld.IncidentParms)
Verse.DebugActionsIncidents/<>c__DisplayClass0_3:<DoTradeCaravanSpecific>b__1 ()
LudeonTK.Dialog_DebugOptionLister:DebugAction (string,single,System.Action,bool)
LudeonTK.Dialog_DebugOptionListLister:DoListingItems (UnityEngine.Rect,single)
LudeonTK.Dialog_OptionLister:DoWindowContents (UnityEngine.Rect)
Verse.Window:InnerWindowOnGUI (int)
UnityEngine.GUI:CallWindowDelegate (UnityEngine.GUI/WindowFunction,int,int,UnityEngine.GUISkin,int,single,single,UnityEngine.GUIStyle)

If someone knows how to solve please do let me know, it's a lot of points for you