I'm doing the extended crew for John Carpenter's "The Thing". I know this movie by heart, and having listened to the commentary and watching the making-of multiple times, know for a fact that Stan Winston did the dog-creature effect. If memory serves me, he was brought in at the last minute to replace someone else who couldn't get it good enough.

In the end credits he is not listed for this, but has a "Special thank you" in it's own spot directly under other effects credits.
I went to include him but recalled the "uncredited" is only on the cast, not the crew.

How should I go about this? I could get timed proof from commentary and documentary about his involvement, or does he just not get a credit because it's not listed (worded really) in the credits for it?

Stan Winston was one of the greatest men in his field.  Unfortunately, we can only deal with what the credits say, not what we want them to say.  In this case, you will have to enter him in the custom field.