Let's break your example down to see what's going on. It's not even clear that Professor B commits any fallacy whatever as he does not state an argument (even a very bad one). Only arguments can be fallacious so the basic question should be just what the argument we are evaluating is.
1) Professor A presents a valid and sound deductive argument in the form of a material conditional."If X has feathers, then X is a bird." (P--->Q). Iff X does have feathers, then the argument amounts to a modus ponens. There is no fallacy but a simple well defined argument form used in basic Propositional Logic. The argument is valid.
2) Professor B is altogether different in your description. He a) believes --"has concluded"-- that the newly discovered animal is a mammal, b) he offers NO argument whatever, but rather c) talks about the classification of fish. This is all we're told.
At first it looks like we've got a fallacy of irrelevance here, one or another variety of a non-sequitur (red herring and ignoratio elenchi were both mentioned). But the simple fact is Professor B never makes any argument attempting to support his belief that P. We're told only that he has "already concluded" that P, and that he "talks about classification of fish." We're not given an argument, however bad, to connect the classification of fish and the conclusion that the new species is mammalian. Thus this is not even a fallacy.
Still, it is worth thinking about just what is going on here because this sort of thing happens in so-called 'arguments' in everyday life and on TV all the time. Sometimes it is purposeful distraction, as when a politician doesn't want to answer a question at a press conference and thus states something which does not even address the topic irrelevantly, but rather replaces the going topic of the questioner with a separate and new topic. Ex. Q."How many soldiers have died in the war this year?" A. "Our soldiers are extremely brave, and we honor each and every one of them. None of their heroic sacrifices are in vain, and we're going to win this war, just you watch! Next question please." Of course no answer was given, so we can't say the answer is irrelevant or poorly argued. It's simple avoidance or else mental incompetence.
Diverting topics in this way is better seen as a rhetorical device than a flaw in reasoning (unless it is done accidentally by someone with cognitive impairment). Whenever politicians are in trouble, they try to "change the narrative" or manipulate the topics that make it onto the front pages of the news. They are practiced in doing so without appearing too conspicuous. It seems that many citizens and voters do not always notice this rhetorical strategy, but it is well worth spotting whenever it occurs. Currently it is more common in public discourse than are any rational tropes!
One last point on the issue of fallacies is that there is no existing theory or even agreed-upon definition of fallacies. So even if there had been a faulty argument made in your example, classifying it with precision and to every logician's satisfaction might well be problematic. See https://plato.stanford.edu/entries/fallacies/#CurIssFalThe for discussion of that topic.