I don't know when you last played if it was before some QOL improvements like showing more mons in the teambuilder or automatically setting max EVs, but I can tell you that there are some forms that still don't appear as legal unless you type them in manually, like Meloetta-Pirouette and the above Wishiwashi-School.
Hemogoblin has gotten some last nerfs both last month and soon to come, going from 99 Atk to 96 Atk, 89 Def to 87 Def, 99 SpA to 96 SpA, and 97 SpD to 89 SpD.
A note on Mega Heracross's speed I want to make is that going minimum speed can theoretically improve the Blissey matchup against suboptimal fast Blisseys, since if Blissey is slower, then it will copy Heracross at +1 and then activate Intrepid Sword itself to get to +2. However, it seems like most Blissey sets are already running minimum speed anyway, which is optimal since in the Imposter ditto, the faster Blissey will transform first and cut its PP down as a result.
sir (Pinsir-Mega) @ Choice Band
Ability: Aerilate
Tera Type: Flying
EVs: 252 HP / 252 Atk / 252 Def / 252 SpA / 252 SpD / 252 Spe
Brave Nature
- Metronome
ufo (Pokestar UFO) @ Clear Amulet
Ability: Delta Stream
Tera Type: Flying
EVs: 252 HP / 252 Atk / 252 Def / 252 SpA / 252 SpD / 252 Spe
Brave Nature
- Metronome
A set I have been trying on ladder is this Mega Pinsir set, more aimed as an experiment in asymmetrical setup against Imposter Blissey but also working out against the occasional Flower Veil or non-tera Heracross and the usual ghosts. My goal is that the Blissey will likely stay in as the Bug/Flying Pinsir, while I terastallize into pure Flying for the super effective advantage. While Blissey can tera itself, I find that most people don't do it, though most of my experience is against the cat bot who never terastallizes, which is the one advantage we humans should remember as long as we have it. You could probably also just run Mega Heracross with Tera Flying itself and achieve similar results, but it is more canon with Pinsir. Tera Flying with Delta Stream has no weak points unlike Ghost which can get hit by heavy damage especially by other STAB Ghost attacks, but is more vulnerable to common Normal chip damage, so it's an interesting tradeoff that I feel could be viable.
EDIT: Here are some comparisons between Heracross and Pinsir. Can Pinsir overthrow Heracross now that they can both change their type?
252+ Atk Choice Band Aerilate Tera Flying Heracross-Mega Double-Edge vs. 252 HP / 252 Def Mew: 399-471 (98.7 - 116.5%) -- 93.8% chance to OHKO
252+ Atk Choice Band Aerilate Tera Flying Pinsir-Mega Double-Edge vs. 252 HP / 252 Def Mew: 464-548 (114.8 - 135.6%) -- guaranteed OHKO
252+ Atk Choice Band Pinsir-Mega Fire Punch vs. 252 HP / 252 Def Mew: 121-143 (29.9 - 35.3%) -- 24.6% chance to 3HKO
252+ Atk Choice Band Heracross-Mega Fire Punch vs. 252 HP / 252 Def Mew: 139-164 (34.4 - 40.5%) -- guaranteed 3HKO
Pinsir beats out Heracross for damage with its own Normal and Flying moves, but falls behind without its super tera STAB boosts otherwise.
Of course, usually Heracross will just run Intrepid Sword to widen the gap instead, but if you used them in the same way then you wouldn't be too far behind with Pinsir. I will also note that Pinsir's base Flying STAB would be more useful than Heracross's Fighting STAB in a Ghost meta.
252 SpA Tera Flying Pinsir-Mega Flamethrower vs. 252 HP / 252 SpD Mew: 50-59 (12.3 - 14.6%) -- possible 7HKO
252 SpA Heracross-Mega Flamethrower vs. 252 HP / 252 SpD Mew: 39-47 (9.6 - 11.6%) -- possible 9HKO
Pinsir's SpA is slightly higher for what it's worth.
252 SpA Mew Flamethrower vs. 252 HP / 252 SpD Tera Flying Pinsir-Mega: 70-83 (20.9 - 24.8%) -- guaranteed 5HKO
252 SpA Mew Flamethrower vs. 252 HP / 252 SpD Tera Ghost Heracross-Mega: 63-75 (17.3 - 20.6%) -- possible 5HKO
252 Atk Mew Double-Edge vs. 252 HP / 252 Def Tera Flying Heracross-Mega: 79-93 (21.7 - 25.5%) -- 0.8% chance to 4HKO
252 Atk Mew Double-Edge vs. 252 HP / 252 Def Pinsir-Mega: 76-90 (22.7 - 26.9%) -- 40.7% chance to 4HKO
Their bulk is actually pretty comparable as well. Even if Pinsir does lose on both ends, it's closer than you might expect within a few single percentages that could even be patched up by a Nature boost if you felt like it.
Here is a replay of Pinsir doing 45% with a Stomp on Ting-Lu.
EDIT:
I was playing some more Pinsir today and I thought that Mega Pinsir may have the strongest Pound/physical Normal moves in the game after seeing
Iron Moth take 81% from Pound, Great Tusk taking 58% from Bind, and taking 60% off random mons with Scratch. That Tera 60BP floor goes a long way.
View attachment 541008
252+ Atk Choice Band Aerilate Tera Flying Pinsir-Mega Pound (60 BP) vs. 252 HP / 252 Def Iron Moth: 264-312 (72.5 - 85.7%) -- guaranteed 2HKO
252+ Atk Choice Band Aerilate Tera Flying Pinsir-Mega Bind (60 BP) vs. 252 HP / 252 Def Great Tusk: 320-380 (73.7 - 87.5%) -- guaranteed 2HKO after trapping damage
Out of all the base -ate ability types (Normal, Fairy, Electric, Ice, Flying), Mega Pinsir has the highest Attack stat of them at 155 (Ursaluna has 140, Iron Valiant/Tapu Bulu have 130, Iron Hands has 140, Baxcalibur has 145).
252+ Atk Choice Band Aerilate Tera Flying Pinsir-Mega Double-Edge vs. 252 HP / 252 Def Mew: 464-548 (114.8 - 135.6%) -- guaranteed OHKO
252+ Atk Choice Band Normalize Tera Normal Ursaluna Double-Edge vs. 252 HP / 252 Def Mew: 430-508 (106.4 - 125.7%) -- guaranteed OHKO
252+ Atk Choice Band Galvanize Tera Electric Iron Hands Double-Edge vs. 252 HP / 252 Def Mew: 430-508 (106.4 - 125.7%) -- guaranteed OHKO
+1 252+ Atk Choice Band Heracross-Mega Double-Edge vs. 252 HP / 252 Def Mew: 333-392 (82.4 - 97%) -- guaranteed 2HKO
Technically Iron Hands can do more with a partner setting up Electric Terrain...
252+ Atk Choice Band Galvanize Tera Electric Iron Hands Double-Edge vs. 252 HP / 252 Def Mew in Electric Terrain: 558-658 (138.1 - 162.8%) -- guaranteed OHKO
And Tera Normal Heracross is stronger but is not real and can't hit ghosts.
+1 252+ Atk Choice Band Tera Normal Heracross-Mega Double-Edge vs. 252 HP / 252 Def Mew: 499-588 (123.5 - 145.5%) -- guaranteed OHKO
EDIT: I have confirmed that black cat no hat runs minimum speed Heracross, as my Pinsir speed tied with it at +2 with 278 speed so it must have had the minimum of 139.
https://replay.pokemonshowdown.com/gen9metronomebattle-1918636504