That episode of the 1999 anime but it's Monster AU. Ballpoint pen as well.

Obligatory monster AU, ballpoint pen.

Look at Killua's smile like he's about to bring some popcorn to watch the show.

Pika and Leorio, ballpoint pen.

Killua on ballpoint pen.

The babies. Tried to simplify Gon to some basic shapes. Ballpoint pen doodles.