My Idea for a MiniBoss in Terraria is the Mighty 'Muu-Shram'. I was bored and though "hey! there is no boss/miniboss themed around the Mushroom Biome!" so Alas! Muu-Shram was created from a figment of my Imagination! My Idea for this Miniboss has two stages, much like the Eye of Cthulhu.
In his first stage, he fires Glowing Mushrooms at the closest player, dealing around 20 damage. I also took a little inspiration from Bowser, otherwise known as "King Koopa" from the Mario Bros. Games. Muu-Shram would spin around in his Glowing Mushroom Shell, bouncing around or if not hitting anything, atomatically changing direction. Dealing roughly 30 damage for each hit, with small knockback.
In his second stage, in which he is at half health, He would shoot, once again, Glowing mushrooms, BUT, these mushrooms instead deal EXPLOSIVE damage. dealing about 40 damage. His Shell Move is also present, however, this time he grows spikes on his Shell and jumps around inside of it, Dealing 60-80 damage for each hit.
Muu-Shram is not meant to be the Hardest Miniboss, just a fun little way of fighting something themed around Glowing Mushrooms. He would have roughly 3000 health, being slightly more than King Slime.
He would drop a small fortune in Glowing Mushrooms, the Trophy he would rarely drop would be his Battle-Scarred Shell, and he could drop really anything themed around Glowing Mushrooms that wasn't a Hammush (too OP for Miniboss drop).
Overall, Muu-Shram is just an Idea from my mind, as I believe the Glowing Mushroom Biome doesn't get the respect it deserves.