Yes, you should revert to the indefinite article:
In the end, it turned out to be a poisonous monkey!
When you add the adjective in that way, it stops referring to the previously known entity that did not have that description.
Unless, of course, you have also previously described that there is a poisonous monkey somewhere, but have simply not yet made the connection between it and the one you found.
I heard on the news there was a poisonous monkey on the loose.
I found a monkey.
The monkey was good . . .
I always went there with the monkey...
What did the monkey like . . .
In the end, it turned out my monkey was the poisonous monkey!
We can use the in this context, because we've also previously used a when referring to the poisonous monkey.
If there had been no previous mention of this, then a would need to be used—but only with that sentence.
For instance, even if no poisonous monkey had been mentioned previously at all, you could use the following sentence:
In the end, it turned out the monkey was poisonous!
This is fine, because of the different construction of the sentence. You're simply assigning another quality to the already mentioned monkey, rather than equating a known monkey with a previously unknown one. (Which is, in fact, the same monkey.)