We weren't expecting it. That's what makes it a surprise. Games Workshop is going to be releasing a new Sons of Horus Praetor mini soon. Have yourselves a look.
From the article:
In this month’s Legion Dispatch, we’re introducing a rather unexpected guest – the Sons of Horus Praetor. If you serve Horus the Warmaster and are looking for a suitably imperious character to command one of his Legion’s armies on the battlefield, then you’ll soon be in luck. In fact, you may already be in luck, but more on that later.
Behold the Sons of Horus Praetor in all his dark glory.