Klunk

I just got this boss through random Brawl. He had around 60 mil HPs and is quite easy.

He dropped [item=151263], coins and random grey stuff.

This is the list of shirts obtained through the Brawler's Guild:

[item=144370]

[item=144377]

Bruce

[item=98082]

[item=151224]

[npc=70666]

[item=98086]

[item=151231]

[npc=70648] & Dippy

[item=98079]

[item=151229]

[npc=70678]

[item=98087]

[item=151232]

[npc=70749]

[item=144365]

[item=144373]

Ash'katzuum

[item=151263]

[item=151264]

Klunk

[item=144368]

[item=144375]

a Seagull

[item=98084]

[item=151221]

[npc=70736]

[item=144371]

[item=144378]

Goredome

[item=98080]

[item=151222]

[npc=70750] & [npc=70748]

[item=144372]

[item=144379]

Unguloxx

[item=98083]

[item=151225]

[npc=71085]

[item=98085]

[item=151230]

[npc=71081]

[item=98081]

[item=151223]

Big Badda Boom

[item=98093]

[item=151238]

[npc=70794]

[item=98092]

[item=151235]

[npc=70616]

[item=144367]

[item=144376]

[npc=70659]

[item=98091]

[item=151233]

[npc=70740]

[item=144366]

[item=144374]

B3@7 B-0X

For completing the achievement Deck Your Collection (Season 2) by defeating several NPCs, you get [item=98543]

Just got this boss through random brawl in Brawler's Guild. He didn't drop anything in particular, just the usual vendor trash and Brawler's Gold.

Edit: As of patch 7.2.5 he drops [item=151263].