I'm using the dev version 1110 of spout and then i get this error when someone kills a monster:
2012-04-19 18:41:42 [SEVERE] Could not pass event EntityDeathEvent to ecoCreature
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:303)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:459)
at org.bukkit.craftbukkit.event.CraftEventFactory.callEntityDeathEvent(CraftEventFactory.java:294)
at net.minecraft.server.EntityLiving.dropDeathLoot(EntityLiving.java:825)
at net.minecraft.server.EntityLiving.die(EntityLiving.java:781)
at net.minecraft.server.EntityLiving.damageEntity(EntityLiving.java:677)
at net.minecraft.server.EntityHuman.attack(EntityHuman.java:750)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:997)
at net.minecraft.server.Packet7UseEntity.handle(SourceFile:33)
at net.minecraft.server.NetworkManager.b(NetworkManager.java:229)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:113)
at org.getspout.spout.SpoutNetServerHandler.a(SpoutNetServerHandler.java:169)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:551)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:449)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
Caused by: java.lang.NoSuchMethodError: org.bukkit.event.Event.(Ljava/lang/String;)V
at se.crafted.chrisb.ecoCreature.events.CreatureKilledByPlayerEvent.(CreatureKilledByPlayerEvent.java:26)
at se.crafted.chrisb.ecoCreature.listeners.ecoEntityListener.onEntityDeath(ecoEntityListener.java:46)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301)
... 16 more

If you want i have 10 more issues like this above but they look the same only another monster name.

Try updated to the latest version here:
http://dev.bukkit.org/media/files/600/502/ecoCreature.jar