Kitty: *reading* I dare Papy and Mettaton to have a child *stops reading* *grins* ROBONES! ROBONES! ROBONES! *snaps fingers*

A skeleton with a robot arm and leg appears (basically, child wasn't born with all of his limbs. Elbow down on left arm, knee down on right leg. Robot parts were made by Alphys and will grow  with him as he gets older. Robones is a pun (?) on robot and bones).

Robones: Uh... dad? Where am I? Who are all these people?

Papyrus: *points to MTT* This is your other dad, Mettaton. *points to Sans* This is your uncle Sans. *points to Toriel* This is your step-aunt Toriel. *points to Asriel* This is your step-cousin Asriel. *points at Blooky* This is your uncle Blooky. *points to the rest of the gang* These are all my friends!

Robones: ...

Asriel: *walks up to Robones* Howdy! I'm Asriel.

Robones: Hi. I'm... Robones. Um... I have a question. How are you a goat if your father is a skeleton? Shouldn't you be a hybrid, kind of like me?

Asriel: *sighs* Sans isn't my actual dad.

Robones: Oh. That would make sense.

Razor: Hi! I'm Razor!

Robones: Hello?

Razor: Are you my friend now?

Robones: Uh... sure?

Razor: Yay! Friend!