Hircocervus

Meaning: as the OED says "A fabulous creature, half goat, half stag." They are of course using fabulous meaning 'out of fable' but I like the ambiguity.

Usefulness: 2 (Perhaps as an insult? Unless you happen to be haunting the halls of Winchester College, of course.)

Logofascination: 2 (The name is from the Latin for billy-goat and stag; another name is the tragelaph, from the Greek for the same creatures.  The genus to which Kudu belong are called Tragelaphus for this reason.)

In the wild: Michael Quinion found it in Umberto Eco, and has written it up here, and as mentioned in both that and the Wikipedia article, the hircocervus comes to us via philosophy. Plato introduced it as an example* and Aristotle takes it further; if you have the brain-power you can read up on the goat-stag in philosophy here.  It also makes a brief appearance in my favourite passage of The Horologicon; the hircocervus is one of the midnight creatures who gather around.

Hoghenhine

Meaning: a house-guest who stays three nights or more; in times past, after the third night the host had the same legal responsibility for them as for any other member of their household.

Usefulness: 1 (I've had house-guests* almost every day this year, and they all stayed at least three nights. I wish I'd known I could have called them hoghenhines.)

Logofascination: 1 (Etymologically, it's a bastardisation of Middle English aȝen hine or oȝen hine - own servant.  There is something appealing in the notion that ancient laws held that three nights was enough to know someone. In a few word books, it has jumped from the legal responsibility for a guest - as for one of your household - to popular definitions like 'one of the family', or 'a member of one's family'.)

In the wild: Mrs Byrne lists it as agenhina, "a guest at an inn who, after having stayed for three nights, was considered one of the family". Karl Hagen does rather a good job of correcting this, although he fails to locate the source of her reference to an inn. Here we turn to historymike who quotes more of the original source than the OED does; The Country Justice says that agenhina "is used in ancient Saxon Laws for him that cometh to an Inne guest-wise".  To give Mrs Byrne her due, she traced her words to original sources wherever possible, and considering she worked on her dictionary during the 1950s and 1960s while touring as a concert pianist, I think she can be forgiven. I do wonder at Eric McKean's "a member of one's family" (Weird and Wonderful Words, 2002), although it is qualified with "chiefly in legal contexts". Even the OED adds "a member of a household; a dependant" to the definition, the semi-colons suggesting additional meanings. There are no citations for these, though, and certainly none for its use to denote a member of one's family. It seems to be one of those words whose usefulness and potential for expanded meanings appeals more to lexicographers than to the populace.

Lacunose

Meaning: having many gaps or hiatuses; used particularly of manuscripts.

Usefulness: 1 (If one were, say, attempting to describe an unexplained gap in blog posts, or a particularly patchy period of posting.)

Logofascination: 1 (Lacuna, meaning a gap in a manuscript - and a number of other things - is from the Latin lacūna, the diminutive of lacus, lake. From lacus we also get lagoon, originally used of pools of water around Venice - Captain Cook was the first to apply it to tropical destinations.)

In the wild: In my apology for being so terribly lacunose? Mind you, I make no rash promises of reformation, but I shall attempt to warn you of any looming lacunae. No lagoons were involved in this one, sadly.