Onix

My name is indiscernable
e'en though my name's quite plain

Onix is derived from the word "onyx"; it's not a difficult name to say, but it is misspelled intentionally.

And if you were to look at me
My color's quite the same

By quite the same, I assume you mean the color is plain as well. Onix is very, very plain (gray).

Attacking me with plain brute force
Is not what you'd desire
You'd probably only hurt yourself
And then invoke my ire

Scratching, Biting, Tackling, or or using untrained physical attacks against Onix would just pit you against his heavy rocklike defenses. You probably don't want to annoy an Onix by doing this, either, because it's huge and can hurt you.

You'd never guess my age if you
would judge me by my size

Onix is really big, but it's only a first evolution, and can go as low as level 5 from an Egg.

To raise me past my current strength
You'd need an ore-like prize

Metal Coat induces its evolution to Steelix.