Grimrock

When you kill a monster, it explodes in a shower of sparks.

I'd like to play this in reverse, so I can spawn a monster while the player is looking.
It would look weird if the monster just appeared out of nowhere.

The zombies have a spawn animation where they crawl out of the ground.
Can it be reversed, so zombies behave like viper roots?

I can't think of any other monsters with spawn animations, are there any?

Animations can be reversed, and exported from Blender, but the shower of sparks played at death, is a particle system.

The zarchtons have also a variation with spawn animation (raising up from the riverside) and the spider (falling fom the ceiling).

The Lava Golem has also one - but AFAIK it is unused in the main game.

I generally spawn the "teleportation_effect" object to do that.

Ive always wanted to reverse the Summon Stone's wake up animation

I too just spawn in front of the player, and play the teleport fx for a while, it looks... passable!

The way particle systems in Grimrock are structured, you actually can reverse them by setting the timescale to a negative number with GameMode.setTimeMultiplier(). Time will appear to rewind for particles until they reach the time they were first emitted. After that, their behaviour becomes very strange.

Needless to say, setting the time multiplier to -1 screws up all manner of other things, like animations and general gameplay, so this isn't practical to actually use in a mod, but it's a funny thing to play with for a minute or two.