diff --git a/tackerclient/common/exceptions.py b/tackerclient/common/exceptions.py
index 28b1509a..a906187a 100644
--- a/tackerclient/common/exceptions.py
+++ b/tackerclient/common/exceptions.py
@@ -148,12 +148,6 @@ class OverQuotaClient(Conflict):
     pass
 
 
-# TODO(amotoki): It is unused in Tacker, but it is referred to
-# in Horizon code. After Horizon code is updated, remove it.
-class AlreadyAttachedClient(Conflict):
-    pass
-
-
 class IpAddressGenerationFailureClient(Conflict):
     pass