I have a question Squirrelloid, how does having Lions, Housecats, Leopards, Cougars and so forth all be f's convey more information than giving each of those creatures a unique icon for their norma living stages and any undead stages, as well as potential war and hunting stages?
Ok, if we went with a rogue-like symbol-mapping, living cats would be 'f'. This instantly tells you its a cat, as opposed to seeing a 'C' and not knowing if its a Camel, Cow, or Cougar. Different cats would have different color 'f's, and eventually, you'd be able to distinguish them immediately based on color. (DF even fails at this - Camels and Cows are the same color, and when those Camels are rather violent migratory animals moving on your map, that difference is a big deal!) Basically, seeing 'f' and knowing "feline" is a lot better than seeing 'E' and not knowing if its an Elephant, a Giant Eagle, or an Elf. There's a big difference between those three things, the difference between felines is a lot smaller.
Typically, rogue-likes make all skeletal creatures 's's, and all zombie creatures 'z's, which is one way of handling this.
Now, DF cares about different creature types overall than rogue-likes do, so we probably don't want to use the same mapping. (There aren't going to be any giant (c)entipedes, so 'c' is actually free, as is 'd' because afaik DF doesn't discriminate between small and large Dragons). Further, some creatures are sufficiently common that they may warrant unique letter assignations.
So, in particular, housecats could probably keep 'c', although if they were given a distinctive 'f' color it would be trivially easy to tell them apart. Dogs could probably keep 'd', although the same applies.
Skeletal or zombie creatures could be bound to 'z' and 's', or bound to the normal group letter with a distinctive color. Ie, define a zombie color and a skeletal color, and if you see that you know its a skeletal or zombie version of something in that group. (This runs into problems if sentient creatures start appearing as skeletons or zombies, since professions span the color map pretty well already, but we could reserve 's' and 'z' for that). War training could be handled similarly. (I doubt there'll be a hunting version of anything except dogs, so that's a moot point).
A general point here is that we'll never eliminate the need to use 'k', but by ensuring similar creatures have similar letter binds we maximize the information you can get before you use 'k', and good use of color will limit the absolute need to use 'k' to distinguish skeletal, zombie, and war creature types within a group.
What other groups we want to define depends on how critical it is we can tell which specific group its a member of, and how many creatures of a given type there is. For example, Equine is a waste of a bind, because there are a mere 2 under Andir's proposal, and the color map is much larger. Ungulate might be too broad, and the Horse itself might be sufficiently common to warrant a unique bind (although doubtful). That said, Mules, Donkeys, Zebra, etc... would all qualify, and could even keep H as (H)ippomorpha, or we could expand to try to identify all Perissodactyla with one symbol. (Relevantly, this would add Rhinoceras to the same letter, and that's about it unless we also have Tapirs some day).
Probable categories: Suggested Character Bind
-Vermin-
non-aquatic Vermin: . (period)
Fish: alpha
Turtle: (keep current, whatever you call that)
-Civs-
Human: p (person)
Elf: e
Goblin: g
Kobold: k
Skeletal Civ Race: s
Zombie Civ Race: z
-Mega and Semi-Mega-
Dragon: incl. in (R)eptile?*
Giant: P (incl. Colossus, Titan, Ettin, Cyclops, possibly Ogres and Trolls)**
Hydra: incl. in (M)onster?
-Animal-
Reptile: R (also bundle amphibians here if there are any non-vermin ones)
Giant Bird: b
Aggressive Fish: F (ie, sturgeon, carp, gar, etc...)
Eel: ~
Shark: S
Marine Mammal: W (for whale, but incl. dolphins/manatees/etc... here)
Pinniped: w (mostly by analogy with W above - walrus, seal, sea lion)
Canines: c (incl. werewolf)
Bears: U (Ursidae)
Feliforma: f (ends up being Felidae, unless someone decides we really need hyenas or something).
Perissodactyla: H (Horse-like. Rhinos will just have to live with it)
Xenartha: X (because Giant Tree Sloths are a good idea!)
Rodentia: r (generally giant versions only. If relevant, include giant lagomorpha here)
Primate: m, A (monkey, Ape)
Pecora: B, D (Bovine, others as 'D'eer - Giraffes can live with it)***.
Pig: Q (it looks like a pig. Also, stick Hippos here - its bad molecular taxonomy, but they look similar morphologically)
Camelid: C
Probiscidians: L (eLephants, could also use M, for 'M'ammoth)
Giant Arachnid: S**** (Spider/Scorpion)
-Other Sentient-
Humanoid: h (snailman, frogman, etc... plus gnomes, and so forth)
Demon: &
Lesser Demon: u (notably fire imp, although fire imp could be assigned to Elemental)
Elemental: E (magma man, blizzard man, fireman, etc...)
-Not (obviously) covered-
foul blendec
giant olm (O is available)
harpy
Iron Man (i suppose it could be a steel grey 'p' or treated as an elemental)
naked mole dog (ok, i don't actually know what this is... i suppose it could be a 'c')
Sasquatch: A
satyr
Sea Monster: M(onster)?
Sea Serpent: M(onster)?
Treant: existing graphic is fine
Satyr, harpy, and foul blendec should share the same letter because they are chimeras. I'd say M, but then it can't be used for M(onster), or some other letter would need to be chosen for those. There are lots of letters left, however, so this shouldn't be a problem to find one for one of these groups.
T is still available for Troll. O could be available for Ogre, or it could be given G (big Goblin). Assuming we think they'd overload P.
*There is only one type of Dragon in DF, so it hardly needs a unique letter
**Giants are P in *bands because humans are p, and giants are large humans.
***Recommend antelopes be treated as 'D'eer and not 'B'ovine despite technically being bovine.
****I realize i'm using S for GCS and Shark, but they're highly unlikely to occur on the same map.