Haven't played in awhile...

[Upinthe]

1) If I have a Horobi, Death's Wail in play can I play a Horobi's Whisper on something with no swamps in play to kill it off with the targeting?

2) I have a [card]Student of the Elements[/card] in play and a [card]Phantasmal Mount[/card]. If I use the Mount to give SotE flying and flip him to Tobita, will I lose the Mount if Tobita dies or vice versa?

3) Can I put 15 [card]Relentless Rats[/card] in my sideboard? It says a deck but not an SB.

[PipOC]

1) Yes, you don't have to control a swamp to play the spell but the destruction(from horobi's whisper) upon it's resolution is conditional upon you controlling a swamp.

2)I don't know

3) Yes, you may have any number of relentless rats between your deck and your sideboard.

[Jebus]

Regarding question 2, the answer is yes.

It changed characteristics, but it is still the same creature.