Some MOD creatures can be ridden and some cannot.
There are problems with the dragons in Dragons Descent and maybe more.

Root level exception in OnGUI(): System.NullReferenceException: Object reference not set to an instance of an object
at Chamber.ChamberPostfix.fix (Verse.Pawn pawn, UnityEngine.Vector3 clickPos, System.Collections.Generic.List`1[Verse.FloatMenuOption]& opts) [0x0007b] in <b5d67f497f3a480ba70951e05feb55c6>:0
at (wrapper dynamic-method) RimWorld.FloatMenuMakerMap.RimWorld.FloatMenuMakerMap.AddHumanlikeOrders_Patch29(UnityEngine.Vector3,Verse.Pawn,System.Collections.Generic.List`1<Verse.FloatMenuOption>)
at (wrapper dynamic-method) RimWorld.FloatMenuMakerMap.RimWorld.FloatMenuMakerMap.ChoicesAtFor_Patch4(UnityEngine.Vector3,Verse.Pawn,bool)
at RimWorld.FloatMenuMakerMap.TryMakeFloatMenu (Verse.Pawn pawn) [0x000dc] in <cd7169108ea74757aa50c5b33d275c15>:0
at (wrapper dynamic-method) RimWorld.Selector.RimWorld.Selector.HandleMapClicks_Patch2(RimWorld.Selector)
at RimWorld.Selector.SelectorOnGUI () [0x00000] in <cd7169108ea74757aa50c5b33d275c15>:0
at RimWorld.MapInterface.HandleLowPriorityInput () [0x0000f] in <cd7169108ea74757aa50c5b33d275c15>:0
at (wrapper dynamic-method) RimWorld.UIRoot_Play.RimWorld.UIRoot_Play.UIRootOnGUI_Patch4(RimWorld.UIRoot_Play)
at (wrapper dynamic-method) Verse.Root.Verse.Root.OnGUI_Patch2(Verse.Root)
UnityEngine.StackTraceUtility:ExtractStackTrace ()
(wrapper dynamic-method) Verse.Log:Verse.Log.Error_Patch2 (string)
(wrapper dynamic-method) Verse.Root:Verse.Root.OnGUI_Patch2 (Verse.Root)