Unknown creatures?

[Decon]

If you remember from the last level in HL2 were you took a tour around citadel in a pod, in one instance you are able to see large brownish creatures that walked on all four legs.
Is this a creature from HL and how come it was never introduced in E1 or E2?

 

[Hectic Glenn]

They are synths. Synths are almost a generic name to creatures that the combine have captured from other planets they have enslaved. They have then modified them from their normal natural phenotype using forceful evolution. This is apparent on the gunship and strider too, you can see organic parts alongside the working parts. They have guns on them and other bits n pieces.

You can have a better look at them if you load their models through the model viewer through SDK. Their purpose in the citadel isn't really known...as they never have an impact on the game, but a lot of this information is explained in 'Raising the Bar'

 

[Druckles]

This was not in Half-Life one. There are actually two creatures we see. They're called "synths". There have been several threads on these creatures here at HalfLife2.net:

do you think these synths..

What's animal do you think would make a good synth?

Synths

Synth Discussion

The two synths that you see are called "Crab synths" and "Mortar synths" and do different things. They are combine, and thus you never saw them in Half-Life.

The reason they weren't introduced in Episode One or Two is that Valve never deemed it necessary to introduce us. We are expecting to see them in Episode Three, however. But considering we were expecting to see them in Episode One and Two, there's no guarantee.

There's a wikipedia entry here and a more specific entry on both the Crab synth and the Mortar Synth in the Half-Life wiki.

You could probably have figured this out for yourself, if you'd looked hard enough, but now you know their names, feel free to look to your heart's content.

Edit: Damnit, Glenn. :|