Where the hard-to-find bosses are
Three bosses players keep asking about go by other names in the game. Zenitsu is Zentaro, Akaza is Akazo, and the umbrella boss is Fujiko. Open your map and switch on Boss Names to see the ones you have already found; the rest show as ???.

Boss markers named on the map: Zentaro by Obari at the edge of Hidden Mist Village, Butterfly Estate and Final Selection on the west side, Rengu in the south, Tengai up in Iceveil Valley.
Zenitsu: Zentaro, The Thunder King
He stands on the west edge of Hidden Mist Village, north of the Hidden Mist shrine and right beside Obari, the Serpent boss. Fast-travel to the Hidden Mist shrine and walk north, or come from Obari and drop down the rocks.
Do not confuse him with the Thunder trainer, who is also named Zentaro but waits on the highest peak above Hidden Mist. The boss is at the bottom; the trainer is at the top.
He has 3,000 health and drops a World Events Chest. His drops and chances are on the bosses page, and what the Thunder route needs from him is on Thunder Breathing. He has been fought in daylight, so you do not need to wait for night.

The drop down to Zentaro, with the area label Wilderness.

The Thunder King Zentaro, 3000 / 3000, World Events Chest. The minimap shows Obari and Zentaro side by side.
Akaza: Akazo, The Shockwave Demon
Start at the Windy Peak shrine and head north-west, to the foot of the snowfield that climbs to Iceveil. The way in is a wall covered in green leaves: it is fake, and you walk straight through it. Akazo waits on the other side.
If someone has just killed him, his name plate shows a countdown, such as Spawns in 2:33, so wait by the wall rather than leaving. He has only been seen fought at night so far; whether he appears by day is not settled.
He drops his top and bottom at 5% each and the Annihilation Type scroll, a Shockwave move. Rolling the Shockwave art itself is covered on the Blood Demon Arts page.

The leafy wall on the way to Akazo.

The Shockwave Demon Akazo, 3000 / 3000, World Events Chest, Akazo's Bottom x1 (5%), Akazo's Top x1 (5%).

The Shockwave Demon Akazo and Spawns in 2:33.
The umbrella boss: Fujiko, The Bladed Wagasa
Fast-travel to Butterfly Estate, cross the red bridge and follow the path to Final Selection Plains. Fujiko is in the far south-west corner, west of the Plains, down the cliff by the water. She has been fought by day and by night.
Watch the level. On her bar, the Bladed Wagasa and Fujiko’s Outfit lines carry a red Lvl. 150 for players below level 150. Whether that stops the drop or only stops you using it is not settled, so farm her at 150 if you can.
The Bladed Wagasa is a Legendary demon weapon that also grants Sun Immunity, so a demon holding it does not burn by day. Her move scroll now shows 10% on the bar, up from 5% at launch. 3,200 health and an Ice Chest; the rest is on the bosses page and the weapon on the weapons page.

The player arrow on an unnamed boss marker in the corner west of Final Selection Plains.

The red bridge out of Butterfly Estate, area label Wilderness.

The Bladed Wagasa Fujiko, 3200 / 3200, Ice Chest: Bladed Wagasa x1 (5%) Lvl. 150, Exp +700, Wen +315, Fujiko's Outfit x1 (1.8%) Lvl. 150, and Shade Breaker x1 (5%) struck through.

The same bar later: no level labels for this player, and Shade Breaker x1 (10%).
Sun Immunity on a weapon means the slot a sun hat would take is free for something else; see the sun hat page and the accessories page.

Bladed Wagasa, Legendary: +2 (Additional Damage), 1.04x, +2.4 (Block Points), Sun Immunity. Source 5% (Fujiko).
The other places people ask about
- Butterfly Mansion is Butterfly Estate in the game, on the west side of the map with its own shrine. It is where the Insect trainer and the breathing training house are; see the map page.
- Muzan wanders instead of standing still: his three night-time areas.
- Every other boss, with health, chest and drop chances, is on the bosses page.