Actually, this opening is interesting. I thought it could be word smithed to bring it more to life, but definitely interesting!
"The Thing kicked sullenly at the floor, and stared around gloomily."
Adverbs are more passive than other types of words and often pain a murkier picture. The thing is sullen and gloomy...can you show us that in a facial expression, a word, or a sigh?
"He sighed, unhappily, running a hand over his scaly arms."
In this case, I think the sigh shows us unhappy, so I'd just cut unhappily.
"He’d had to come through there at one point, but when?"
How about "He must have come..." to eliminate the had had?
"And when would he get back out through there again? What was on the other side of the Door?"
I like the capitalization of Door -- nice effect. I'd also capitalize the The.
" There were others over there, he knew."
Since we're in his POV, the "he knew" is redundant.
"Other living creatures that screamed and called him ‘thing’. "
Before this I was wondering if he really thought of himself as The thing but I saw here that he probably does. I suggest, though, calling him "Thing" instead of "The Thing" to really give us a sense from the start that this is the name he thinks of himself by.
"He was bored. Painfully, utterly, completely bored.When had he not been bored?"
This was, IMO, the weakest part of the opening. It seemed to shine light on the fact that nothing has really happened yet, a fact that until this line, I had not really noticed because I was interested in Thing's situation. Plus, it's telling. You already showed us Thing's boredom. Trust yourself.