Luna Bold, Your Local Kobold

Yoshi's New Island contains a "bad ending" that is seen if a player finishes at least one level using the Flutter Wings assist item and then does not go back to finish it unassisted before fighting the final boss.

Normally, the Baby Bowser boss battle is followed by adult Bowser "suddenly warping through space and time" to challenge Yoshi. However, if the Flutter Wings were used, the game simply ends after the Baby Bowser battle, the credits are shown as usual, and Bowser rips apart the final image at the end before telling the player to go back and finish all levels without the Flutter Wings.

One of Yoshi's transformations in Yoshi's Woolly World is Mega Yoshi, whereby he simply becomes very large, similarly to Mega Mario in New Super Mario Bros. He does not change his design and continues to use his regular model, merely scaled to a larger size.

However, in the game's files, an early design for Mega Yoshi can be found that is actually different from regular Yoshi, making him stand less upright and thus giving him more of a Super Mario World-inspired appearance.

So, for non-SDV players, in most farming sims you can get married to certain characters and have children with them, however this is typically a permanent choice that can't be undone, so if you want to marry someone else or not have kids you would have to start over with a fresh save.

But SDV is different. The game has a witch's hut you can eventually unlock that lets you use...black magic, basically, to soft-reset marriage/children choices without starting over. There's a divorce option so you can ditch your spouse, but that's the only normal one. It also adds an option to turn your children into doves to get rid of them, hence OP saying they "birded" their kids. You're not...technically killing them...but only barely.

(It also adds an option to mind-wipe your previous spouse so they don't remember being married to you and thus don't hate you, and you can even remarry them then if you want, but I don't think that was needed for this, just figured it was sinister enough to warrant a mention.)

All that to say, the reddit OP married a character, progressed to the point that they had a child with them, and then used black magic to "kill" the child and divorce their spouse before moving on to the next marriage candidate and doing the same, over and over until they had gone through all twelve marriage candidates, resulting in a collection of twelve "dead" children.

is some uniquely unhinged

behavior indeed.