Here is what I found that is very strongly the basis for the concept of the hydra.
Here is the actual Hydra Bacteria:
As you can see it bares a similar resemblance to the hydra seen in the e3 video. Furthermore if one looked at the e3 video you could see its other parts. One of which looked very similar to this:
Only it was one tentacle thing. This is a colony of them.
Just found it interesting how Valve based it off of pre-existing species of terrestrial bacteria.
Here is the actual Hydra Bacteria:

As you can see it bares a similar resemblance to the hydra seen in the e3 video. Furthermore if one looked at the e3 video you could see its other parts. One of which looked very similar to this:

Only it was one tentacle thing. This is a colony of them.
Just found it interesting how Valve based it off of pre-existing species of terrestrial bacteria.