diff --git a/data/json/monstergroups/misc.json b/data/json/monstergroups/misc.json
index c57661eeec25b..8e6749b8dd36b 100644
--- a/data/json/monstergroups/misc.json
+++ b/data/json/monstergroups/misc.json
@@ -509,6 +509,12 @@
     "name": "GROUP_JABBERWOCK",
     "monsters": [ { "monster": "mon_jabberwock" } ]
   },
+  {
+    "name": "GROUP_RATTLESNAKE",
+    "type": "monstergroup",
+    "monsters": [ { "monster": "mon_rattlesnake", "weight": 19 }, { "monster": "mon_rattlesnake_big", "weight": 1 } ],
+    "//": "Normally for the time until mutants start spawning we take the growth time of a mutant baby with default evolution rates. But because rattlesnakes follow the mutation logic of invertebrates, we assume there were some mildly oversized variations at the start of the Cataclysm already."
+  },
   {
     "type": "monstergroup",
     "name": "GROUP_CAMP_HUNTING",
diff --git a/data/json/monstergroups/mutant_vertebrate_reproduction.json b/data/json/monstergroups/mutant_vertebrate_reproduction.json
index aca101eebf4c2..bb96d470a1664 100644
--- a/data/json/monstergroups/mutant_vertebrate_reproduction.json
+++ b/data/json/monstergroups/mutant_vertebrate_reproduction.json
@@ -12,6 +12,6 @@
   {
     "name": "REPRODUCTION_MUT_RATTLESNAKE",
     "type": "monstergroup",
-    "monsters": [ { "monster": "mon_moose_calf", "weight": 19 }, { "monster": "mon_tusked_moose_calf", "weight": 1 } ]
+    "monsters": [ { "monster": "mon_rattlesnake_s", "weight": 19 }, { "monster": "mon_rattlesnake_big_s", "weight": 1 } ]
   }
 ]
diff --git a/data/json/monstergroups/wilderness.json b/data/json/monstergroups/wilderness.json
index 9c06355f99f93..c4ca9ecbeb7af 100644
--- a/data/json/monstergroups/wilderness.json
+++ b/data/json/monstergroups/wilderness.json
@@ -179,7 +179,7 @@
     "default": "mon_null",
     "is_animal": true,
     "monsters": [
-      { "group": "GROUP_DEER_MUT_SPIDERS", "weight": 4, "cost_multiplier": 2, "pack_size": [ 1, 3 ] }
+      { "group": "GROUP_DEER_MUT_SPIDERS", "weight": 4, "cost_multiplier": 2, "pack_size": [ 1, 3 ] },
       { "monster": "mon_bear_mutant_3headed", "weight": 1, "cost_multiplier": 10 }
     ]
   },
@@ -221,7 +221,7 @@
     "default": "mon_null",
     "is_animal": true,
     "monsters": [
-      { "monster": "mon_rattlesnake", "weight": 3 },
+      { "group": "GROUP_RATTLESNAKE", "weight": 3 },
       { "monster": "mon_gartersnake", "weight": 10 },
       { "monster": "mon_wormsnake", "weight": 5 },
       { "monster": "mon_skink_fivelined", "weight": 2, "pack_size": [ 1, 2 ] }
diff --git a/data/json/monsters/reptile_amphibian.json b/data/json/monsters/reptile_amphibian.json
index 8ec941a5786c6..8644f19094403 100644
--- a/data/json/monsters/reptile_amphibian.json
+++ b/data/json/monsters/reptile_amphibian.json
@@ -130,7 +130,7 @@
     "special_attacks": [ [ "RATTLE", 6 ] ],
     "anger_triggers": [ "PLAYER_CLOSE" ],
     "fear_triggers": [ "HURT" ],
-    "reproduction": { "baby_monster": "mon_rattlesnake_s", "baby_count": 10, "baby_timer": 135 },
+    "reproduction": { "baby_monster_group": "REPRODUCTION_MUT_RATTLESNAKE", "baby_count": 10, "baby_timer": 135 },
     "baby_flags": [ "SUMMER" ],
     "//babies": "On average, a female rattlesnake gives birth to at least eight babies, up to around fifteen.  Most baby rattlesnakes are born in the early summer, but sometimes in late summer and early autumn.",
     "extend": { "flags": [ "BADVENOM" ] }