What are some situation-modifying monster abilities you've really found enjoyable?

Long story short, I'm homebrewing Sheeptar the Sheep King from Sword Art Online Abridged, the fluffy sheep-xenomorph that's able to climb walls and spit acid. And I'm trying to find inspiration with non-magic monster abilities. So whether a monster was able to create terrain that changed the battle dynamics, your DM found a really cool way to user spiderclimb that changed your approach (but, if possible, didn't put your melee fighters in a total lurch), or other kind of things I'm interested to hear.

Basically just getting those inspirations to further my development. Since right now my ideas are mostly: able to move real fast, do damage, and make an acid splash.