Why is Professor Slughorn in Slytherin

Is Horace Slughorn boss of Slytherin during his second term at Hogwarts?

Presumably (but not irrevocably - Wikia is wrong as usual and has presented unreferenced information as a "fact") was the head of the house, because as the head of house while When Snape's flight met, Slughorn was there.

"Minerva!" said a squeaky voice and looked behind her, still protecting Luna from flying spells. Harry saw Professor Flitwick and Sprout in their nightwear sprint down the corridor towards them, the huge Professor Slughorn panting in the back. ( DH, Ch 30 - that's even BEFORE Snape escapes and stops being Headmaster while fighting McGonagall)

... and later Minerva also seems to act as host towards Slughorn:

"I'll be waiting for you and the Slytherins in the Great Hall in twenty minutes," said Professor McGonagall. “If you want to go with your students, we won't stop you. But if any of you try to sabotage our resistance or take up arms against us in this castle, then we will duel, Horace, to kill. "
"Minerva!" he said horrified.
"It is time for Slytherin House to decide his loyalty," interrupted Professor McGonagall. "Go and wake your students, Horace."

He later returns after bringing out his students and dueling Voldemort (with McGonagall and Kingsley).

Additionally, after Dumbledore was killed by HBP in the end, he was asked by the incumbent Headmistress McGonagall to represent his house with other householders:

"Thank you, Hagrid," said Professor McGonagall, getting up immediately and turning to face the group around Bill's bed. “I need to see the Ministry when they're here. Hagrid, please say that House people - Slughorn can represent Slytherin - that I want to see her in my office right away. I want you to join us too. '( Half-Blood Prince, chapter 29, "The Phoenix Lamentation" )

All in all, although it is never specifically stated, see we act him and be positioned as if he is the master of the house .


TO UPDATE

Since the question was not clear, I covered the 7th year (Deathly Hallows).

In year 6 (HBP), Snape is undoubtedly the head of the Slytherin house:

"I'm his Head of House and I'll decide how hard or otherwise it should be," Snape said curtly. "Follow me, Draco." ( Half-Blood Prince, Chapter 15: The Unbreakable Vow )

DVK-on-Ahch-To

I haven't reviewed Pottermore, but I don't think there is anything relevant to add.

TGnat

I think the operation is asking about Slughorn's position during HBP.

DVK-on-Ahch-To

@TGnat - I asked the OP but also added HBP information for now

James C.

@ DVK - Doesn't matter if I ask what is OP? =)

DVK-on-Ahch-To

@ JamesC - Original Poster. Whoever asked the question