"It is probably a moose."
Translation:Det är troligtvis en älg.
Troligtvis är det en älg is ok. möjligtvis means 'possibly', that's less likely than 'probably'.
If you say probably, you're not sure but you think the chance is pretty big that it is a moose.
If you say possibly, you're just open to the possibility that it could be the moose, but you didn't say you think there's a big chance it is.
When you say 'What is it?', it is unknown what 'it' refers to, so we say Vad är det?. Then when you answer that question, you repeat the same det, because you're answering what the previously unknown det is. So the dialogue would be Vad är det? Det är en älg. If you use det där ('that') or det här ('this'), it works the same way – only the -t versions work in this dialogue.
Thanks for your explanation. Is it usually used by swedish people in the correct way or do they also mix it up? (It think about the German "scheinbar" and "anscheinend". Both have a similar meaning and if you are aware of the difference, it really has an impact which one you use. But most Germans use these words without thinking about their difference.)
I think people probably confuse those two in German because they have the same root with schein, but möjligtvis and troligtvis don't have the same root at all so it's not the same thing. Of course people will confuse anything and everything anyway, but I've never noticed anyone mixing up these two. On the other hand, a lot of people have trouble keeping words like ovärderlig (invaluable) and värdelös (worthless) apart.