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Thalacephalos is Lieutenant Blouse's horse. Blouse believes it to be a stallion and names it after the stallion of Discworld's most famous general, Tacticus. Like all the other members of Blouse's regiment, perception and reality are far apart and Thalacephalos is a female like the other recruits. Thalecephalos is a very evil tempered and sneaky mare who tries to step on Polly's feet when she is acting as batman for Blouse and,when thwarted because Polly is use to miserable horses, bites Polly in the 'socks' which causes Blouse to faint and fall off the horse - he thinking that she has been bitten in that sensitive part of the male anatomy rather than in the socks she has stuffed there.

Tacticus is patterned after Alexander the Great, it is not surprising to find that Thalacephalos has a connection to the Roundworld figure as well since Bucephalos ("Oxhead") was the mount of Alexander the Great. Thalacephalos has three possible meanings: Cephalo means "head" in Greek so combined with:

1) Greek Thalame meaning "den" or "lair", "cave", thus "thala-" meaning "empty" = empty-headed;

2) "Thallus" meaning a plant body = stem-head; or

3) Thallus standing in for "Phallus" meaning "penis" = dick-headed.

It is up to the reader to decide which Pratchett was thinking of.