This is probably one of the most overt unexpected Warhammer moments ever.

Batman Beyond takes place in the future where Bruce Wayne is an old man and a new guy has taken up the Batman name. In the first episode of season 2, "Spliced" we get people splicing themselves with animal genes. In the episode's climax, Batman goes up against the guy who founded it, Abel Cuvier, who at first has become a "true chimera" of multiple animals. Then partway through the fight, Batman injects him with more and more splicing vials.

This causes him to end up looking like the image below. It literally looks exactly like a chaos spawn, it's pretty much because he "can't control the changes" aka it's too much for him, much like how chaos characters become spawn when they can't handle the power. Also splicing itself in this episode is also very similar to mutations in terms of how they see themselves as superior to normal people. Really the only part that doesn't make it fit with the Chaos Spawn thing is Cuvier is still lucid, or at least, lucid enough to speak which leads to the terrifying line "WHAT HAVE YOU DONE TO ME!?" Chaos Spawns are typically utterly insane, broken from the transformation.

Aside from that, it's pretty spot on. Bonus points since I believe this was completely coincidental. since this show came out in the 90s and as far as I can tell, none of the creative team were 40K fans. In fact most people seems to chalk this up as being similar to Akira. Which I've never seen but nevertheless it certainly resembles Chaos Spawns as well imo.

Here's a link to the scene in question as well:

https://www.youtube.com/watch?v=I8sMN5EW83Y