Ghost crabs (PleasingFungus, floatingatoll).

They have stats similar to fire crabs but slightly stronger. Instead
of fire clouds, they breathe clouds of ghostly flame, the spell that
revenants have.

I think that they combine two fairly unused monster mechanics -- the
"big cloud" that crabs have and the otherwise-unused ghostly flame --
to create a monster that is fairly understandable based on previous
ones but has interesting gameplay. Unlike revenants, they are not
spellcasters, and the cloud is more about bringing fellow monsters
to beat you up, since it does less damage, their HD being lower. They
also contribute a few more non-undead to Crypt, which I think is good
to have.

I'm not certain about the spawn weight, but in tests they seemed to show
up once or twice a level on Crypt:1 which seems about right. They simply
replace mummies in Tar; this doesn't seem very controversial.

1 files changed, 5 insertions, 0 deletions

diff --git a/crawl-ref/source/enum.h b/crawl-ref/source/enum.h
index c38e74f7b1..8af7d2cf81 100644
--- a/crawl-ref/source/enum.h
+++ b/crawl-ref/source/enum.h
@@ -2429,6 +2429,9 @@ enum monster_type                      // menv[].type
 #if TAG_MAJOR_VERSION == 34
     MONS_HOMUNCULUS,
     MONS_SOUPLING,
+#else
+    MONS_GHOST_CRAB,
+    MONS_CRAB,
 #endif
 
     MONS_BUTTERFLY,
@@ -3199,6 +3202,8 @@ enum monster_type                      // menv[].type
     MONS_GUARDIAN_GOLEM,
     MONS_SPELLFORGED_SERVITOR,
     MONS_OCTOPODE_CRUSHER,
+    MONS_CRAB,
+    MONS_GHOST_CRAB,
 #endif
 
     NUM_MONSTERS,               // used for polymorph