I didn't know that - I had a "ckeditor" package installed on my 
server, which was started from clean recently and only has Horde, 
Apache, Postfix and Dovecot installed. Something must have pulled that 
in as a dependency, maybe because I chose to use the Ubuntu 
"php-horde-wemail" package to install Horde, rather than Pear. In any 
case, the symlink in horde/js/ pointing to ckeditor_basic.js in 
/usr/share/javascripts/ckeditor was broken in my installation as no 
such file existed. Trying to use the HTML compose in IMP jsut gave a 
me a "Loading...", and in the JS console I had an error about ckeditor 
being undefined - which led me to the solution: downgrading the 
installed version of ckeditor to the previous one fixed the problem 
and no other action was needed.

In the process I looked at the ckeditor website and saw that the 
current version has ditched the different "modes" in favour of letting 
the user customise ckeditor.js before downloading, a bit like how 
jQuery do it. This explains why the _basic file was missing in the 
installed version: it's no longer part of the ckeditor package.

It may be that no action is needed from Horde team, but I thought I'd 
report and let you be the judge. My apologies if I've wasted your time.

Horde bundles it's own copy of ckeditor in the Horde_Editor package. 
Not sure what Ubuntu's ckeditor package has to do with Horde.

On a fresh install of Horde on Ubuntu 14.04 I was unable to get the 
HTML composition to work. Some digging revealed that the symlinked 
script "ckeditor_basic.js" was missing from my ckeditor installation. 
Apparently ckeditor has switched to a new file organisation with 
version 4 and "ckeditor_basic.js" is only available in versions below. 
I had to downgrade to Ubuntu 13.10's version (which is 3.6.1-1) to get 
HTML composition to work.

Apologies if I'm posting this in the wrong queue - I'm new to Horde 
and not sure how extensively ckeditor is used by the different 
applications. Maybe this belongs in the IMP queue...