diff --git a/neutronclient/common/exceptions.py b/neutronclient/common/exceptions.py
index 9a12f1995..9728fbdd7 100644
--- a/neutronclient/common/exceptions.py
+++ b/neutronclient/common/exceptions.py
@@ -148,6 +148,12 @@ class OverQuotaClient(Conflict):
     pass
 
 
+# TODO(amotoki): It is unused in Neutron, but it is referred to
+# in Horizon code. After Horizon code is updated, remove it.
+class AlreadyAttachedClient(Conflict):
+    pass
+
+
 class IpAddressGenerationFailureClient(Conflict):
     pass