掲示板 Forums - What does した mean?

I'm working my way through a children's light novel, and in the first two pages I've seen した used 3 times. No kanji (though the book omits a fair bit of kanji so I guess it could have some), no noticeable grammar structure around it, just slapped in the middle of sentences (comes before a noun twice, and then the third time is before an unidentifiable hiragana wad of "らなかなか"). What exactly does this した mean?

I probably shoulda added more context, whoops:

1. うっそうとした林をバタバタと駆けていく, ひとりと一匹。

2. 意志の強そうな目をした少年 [character name]。

3. [Creature name]は一度走りだしたらなかなか止まらないモンスターだ。

I get what the first two sentences are saying (aside from that weird くin 駆けていく) aside from the strange した, whereas the third I'm thoroughly baffled on altogether, but I thought the した issue would be a good thing to start with since it seems so common. Past of する was my original guess, I just didn't see how that really fit and also thought it had to be at a sentence or clause end.

Ohhh, thank you!! Way more stuff makes sense now (I thought that one thing was just 駆けている conjugated with a random く. Oops!)

I can't find anything on renshuu about をした or をしている to see exactly what they are and how they work though, unless I'm just looking in totally the wrong places. Is there somewhere I can see exactly how that stuff comes together and works?