Changelings can only fly whilst disguised if they're disguised as something that can fly

src/main/java/com/minelittlepony/unicopia/player/IFlyingPredicate.java

@@ -0,0 +1,10 @@
+package com.minelittlepony.unicopia.player;
+
+/**
+ * Predicate for abilities to control whether a player can fly.
+ *
+ * This overrides what the race specifies.
+ */
+public interface IFlyingPredicate {
+    boolean checkCanFly(IPlayer player);
+}

src/main/java/com/minelittlepony/unicopia/player/IGravity.java

@@ -1,6 +1,6 @@
 package com.minelittlepony.unicopia.player;
 
-public interface IGravity {
+public interface IGravity extends IFlyingPredicate {
     boolean isFlying();
 
     float getFlightExperience();

src/main/java/com/minelittlepony/unicopia/player/IPlayer.java

@@ -14,6 +14,7 @@ import net.minecraft.entity.Entity;
 import net.minecraft.entity.player.EntityPlayer;
 import net.minecraft.item.ItemFood;
 import net.minecraft.item.ItemStack;
+import net.minecraft.server.MinecraftServer;
 import net.minecraftforge.fml.common.FMLCommonHandler;
 
 public interface IPlayer extends ICaster<EntityPlayer>, IRaceContainer<EntityPlayer>, ITransmittable, IPageOwner {
@@ -63,7 +64,13 @@ public interface IPlayer extends ICaster<EntityPlayer>, IRaceContainer<EntityPla
     void beforeUpdate(EntityPlayer entity);
 
     static EntityPlayer getPlayerFromServer(UUID playerId) {
-        Entity e = FMLCommonHandler.instance().getMinecraftServerInstance().getEntityFromUuid(playerId);
+        MinecraftServer server = FMLCommonHandler.instance().getMinecraftServerInstance();
+
+        if (server == null) {
+            return UClient.instance().getPlayerByUUID(playerId);
+        }
+
+        Entity e = server.getEntityFromUuid(playerId);
 
         if (e instanceof EntityPlayer) {
             return (EntityPlayer)e;

src/main/java/com/minelittlepony/unicopia/player/PlayerGravityDelegate.java

@@ -3,6 +3,7 @@ package com.minelittlepony.unicopia.player;
 import com.minelittlepony.unicopia.Race;
 import com.minelittlepony.unicopia.USounds;
 import com.minelittlepony.unicopia.mixin.MixinEntity;
+import com.minelittlepony.unicopia.spell.IMagicEffect;
 import com.minelittlepony.unicopia.util.serialisation.InbtSerialisable;
 
 import net.minecraft.block.Block;
@@ -35,13 +36,28 @@ class PlayerGravityDelegate implements IUpdatable<EntityPlayer>, IGravity, InbtS
         this.player = player;
     }
 
+    public boolean checkCanFly(IPlayer player) {
+        if (player.getOwner().capabilities.isCreativeMode) {
+            return true;
+        }
+
+        if (player.hasEffect()) {
+            IMagicEffect effect = player.getEffect();
+            if (!effect.getDead() && effect instanceof IFlyingPredicate) {
+                return ((IFlyingPredicate)effect).checkCanFly(player);
+            }
+        }
+
+        return player.getPlayerSpecies().canFly();
+    }
+
     @Override
     public void onUpdate(EntityPlayer entity) {
 
-        entity.capabilities.allowFlying = entity.capabilities.isCreativeMode || player.getPlayerSpecies().canFly();
+        entity.capabilities.allowFlying = checkCanFly(player);
 
         if (!entity.capabilities.isCreativeMode) {
-            entity.capabilities.isFlying |= entity.capabilities.allowFlying && isFlying && !entity.onGround;
+            entity.capabilities.isFlying |= entity.capabilities.allowFlying && isFlying && !entity.onGround && !entity.isWet();
         }
 
         isFlying = entity.capabilities.isFlying && !entity.capabilities.isCreativeMode;

src/main/java/com/minelittlepony/unicopia/player/PlayerSpeciesList.java

@@ -2,6 +2,8 @@ package com.minelittlepony.unicopia.player;
 
 import java.util.UUID;
 
+import javax.annotation.Nullable;
+
 import com.minelittlepony.unicopia.Race;
 import com.minelittlepony.unicopia.UConfig;
 import com.minelittlepony.unicopia.forgebullshit.FBS;
@@ -66,6 +68,7 @@ public class PlayerSpeciesList {
         throw new IllegalArgumentException("entity");
     }
 
+    @Nullable
     public IPlayer getPlayer(EntityPlayer player) {
         if (player == null) {
             return null;
@@ -74,6 +77,7 @@ public class PlayerSpeciesList {
         return FBS.of(player).getPlayer();
     }
 
+    @Nullable
     public IPlayer getPlayer(UUID playerId) {
         return getPlayer(IPlayer.getPlayerFromServer(playerId));
     }

src/main/java/com/minelittlepony/unicopia/spell/SpellDisguise.java

@@ -7,22 +7,25 @@ import javax.annotation.Nullable;
 
 import com.minelittlepony.unicopia.Race;
 import com.minelittlepony.unicopia.UClient;
-import com.minelittlepony.unicopia.mixin.MixinEntity;
+import com.minelittlepony.unicopia.player.IFlyingPredicate;
 import com.minelittlepony.unicopia.player.IOwned;
 import com.minelittlepony.unicopia.player.IPlayer;
 import com.minelittlepony.unicopia.player.PlayerSpeciesList;
 import com.mojang.authlib.GameProfile;
 
 import net.minecraft.entity.Entity;
+import net.minecraft.entity.EntityFlying;
 import net.minecraft.entity.EntityList;
 import net.minecraft.entity.EntityLiving;
 import net.minecraft.entity.EntityLivingBase;
+import net.minecraft.entity.boss.EntityDragon;
+import net.minecraft.entity.passive.EntityAmbientCreature;
 import net.minecraft.entity.player.EntityPlayer;
 import net.minecraft.inventory.EntityEquipmentSlot;
 import net.minecraft.item.ItemStack;
 import net.minecraft.nbt.NBTTagCompound;
 
-public class SpellDisguise extends AbstractSpell {
+public class SpellDisguise extends AbstractSpell implements IFlyingPredicate {
 
     @Nonnull
     private String entityId = "";
@@ -302,4 +305,28 @@ public class SpellDisguise extends AbstractSpell {
             }
         }
     }
+
+    @Override
+    public boolean checkCanFly(IPlayer player) {
+        if (!player.getPlayerSpecies().canFly()) {
+            return false;
+        }
+
+        if (entity != null) {
+            if (entity instanceof EntityFlying
+                    || entity instanceof net.minecraft.entity.passive.EntityFlying
+                    || entity instanceof EntityDragon
+                    || entity instanceof EntityAmbientCreature) {
+                return true;
+            }
+
+            if (entity instanceof IOwned) {
+                IPlayer iplayer = PlayerSpeciesList.instance().getPlayer(IOwned.<UUID>cast(entity).getOwner());
+
+                return iplayer != null && iplayer.getPlayerSpecies().canFly();
+            }
+        }
+
+        return false;
+    }
 }