Well one way you could do it is that you need to lay eggs, and let them hatch. Then you can use them to say buy new tentacles or tentacle improvements... bumps, knobs, spikes, etc...
So... basically a texture customizer for 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.