diff --git a/swh/model/hypothesis_strategies.py b/swh/model/hypothesis_strategies.py
--- a/swh/model/hypothesis_strategies.py
+++ b/swh/model/hypothesis_strategies.py
@@ -28,12 +28,20 @@
 )
 
 from .from_disk import DentryPerms
-from .identifiers import identifier_to_bytes, snapshot_identifier
+from .identifiers import (
+    EXTENDED_SWHID_TYPES,
+    SWHID_TYPES,
+    CoreSWHID,
+    ExtendedSWHID,
+    identifier_to_bytes,
+    snapshot_identifier,
+)
 from .model import (
     BaseContent,
     Content,
     Directory,
     DirectoryEntry,
+    ExtID,
     ObjectType,
     Origin,
     OriginVisit,
@@ -143,6 +151,34 @@
 )
 
 
+def core_SWHID_d():
+    return builds(
+        dict,
+        namespace=just("swh"),
+        scheme_version=just(1),
+        object_type=sampled_from(SWHID_TYPES),
+        object_id=sha1_git(),
+    )
+
+
+def core_SWHID():
+    return core_SWHID_d().map(lambda d: CoreSWHID(**d))
+
+
+def extended_SWHID_d():
+    return builds(
+        dict,
+        namespace=just("swh"),
+        scheme_version=just(1),
+        object_type=sampled_from(EXTENDED_SWHID_TYPES),
+        object_id=sha1_git(),
+    )
+
+
+def extended_SWHID():
+    return extended_SWHID_d().map(lambda d: ExtendedSWHID(**d))
+
+
 def origins_d():
     return builds(dict, url=urls())
 
@@ -290,6 +326,19 @@
     return one_of(present_contents(), skipped_contents())
 
 
+def extids_d():
+    return builds(
+        dict,
+        target=extended_SWHID().map(str),
+        extid_type=sampled_from(["hg", "git", "bzr"]),
+        extid=sha1(),
+    )
+
+
+def extids():
+    return extids_d().map(ExtID.from_dict)
+
+
 def present_contents_d():
     return builds(
         dict,
@@ -424,6 +473,7 @@
         ("release", releases),
         ("revision", revisions),
         ("directory", directories),
+        ("extid", extids),
     ]
     if split_content:
         strategies.append(("content", present_contents))
@@ -457,6 +507,7 @@
         ("release", releases_d),
         ("revision", revisions_d),
         ("directory", directories_d),
+        ("extid", extids_d),
     ]
     if split_content:
         strategies.append(("content", present_contents_d))
diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -39,7 +39,7 @@
     pass
 
 
-KeyType = Union[Dict[str, str], Dict[str, bytes], bytes]
+KeyType = Union[Dict[str, str], Dict[str, bytes], Tuple[Union[str, bytes], ...], bytes]
 """The type returned by BaseModel.unique_key()."""
 
 
@@ -1093,3 +1093,25 @@
                 d[swhid_key] = CoreSWHID.from_string(d[swhid_key])
 
         return super().from_dict(d)
+
+
+@attr.s(frozen=True, slots=True)
+class ExtID(BaseModel):
+    object_type: Final = "extid"
+
+    extid = attr.ib(type=bytes, validator=type_validator())
+    extid_type = attr.ib(type=str, validator=type_validator())
+    target = attr.ib(type=ExtendedSWHID, validator=type_validator())
+
+    @classmethod
+    def from_dict(cls, d):
+        return super().from_dict(
+            dict(
+                extid=d["extid"],
+                extid_type=d["extid_type"],
+                target=ExtendedSWHID.from_string(d["target"]),
+            )
+        )
+
+    def unique_key(self) -> KeyType:
+        return attr.astuple(self)
diff --git a/swh/model/tests/test_hypothesis_strategies.py b/swh/model/tests/test_hypothesis_strategies.py
--- a/swh/model/tests/test_hypothesis_strategies.py
+++ b/swh/model/tests/test_hypothesis_strategies.py
@@ -32,6 +32,7 @@
     "revision",
     "directory",
     "content",
+    "extid",
 )
 
 
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -1119,3 +1119,18 @@
             ),
             **_common_metadata_fields,
         )
+
+
+@given(strategies.extids())
+def test_extid(extid):
+    assert extid
+    assert extid.unique_key()
+    assert extid.target.object_type.value in (
+        "cnt",
+        "dir",
+        "rev",
+        "rel",
+        "snp",
+        "ori",
+        "emd",
+    )