Sounds cool, so the eggs act like a sort resource/currency you use in the shop to buy upgrades. Bumps and spikes would be awesome customization for the tentacles
Doesn't need to be a monster, when it thematically wouldn't make sense.
How about you have a groper tentacle (style thing) slowly emerge from the hole, and then disappear below. So it would then need a backend as well, but you'd then be reusing an existing mesh/texture, just used in a different manner. Maybe a little smaller or several escaping at once.