Ao the Overkitty said:
Is there a cat familiar named Chester?
No... but during the same adventure they disrupted a pair of weird diabolists who were sacrificing a Tanil's Fox (from Creature Collection). It sort of got bonded to one of the PCs when he was brought to negative hps.
Thanks for the responses! This is my first time DMing (and apparently last time until I find a new group in Victoria), so it's interesting to hear the input of more experienced players and DMs.
I see what you all mean about not asking them beforehand if they had any problems with me modifying their characters. I've been pretty good so far at asking them what sorts of things they want to do, but I decided not mention this one because I didn't want to spoil the surprise. I did manage to surprise them

, but ultimately I think they were disappointed to not be in control of the development of their characters.
This discussion made me think of Greg Kostikyan's article on game design,
I Have No Words And I Must Design. In RPGs, the player's character is their only way of interacting with the game, hence the complicated rules involved with their creation and for performing actions. By forcing a change on the PCs like I did, I violated the sanctity of their in-game actor. If they'd given me permission for modification beforehand, at least they'd have had an opportunity to prepare themselves mentally for the changes necessary in their outlooks.
Next time I'll make sure that the players at least understand what sorts of things I might spring on their characters!
-blarg