I'm glad you asked. Truly rule34.xxx doesn't offer a built in solution to do so. But worry not, it is possible through Google search.
Just type the following in Google (and replace the first word with your search term obviously):
hornyyy "rule34.xxx/index.php?page=pool*"
beast "rule34.xxx/index.php?page=pool*"
Its not perfect, but its good enough to search for a tag or the user/creator of a pool.
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Similar this works on other search sites. E.g. this parameter works on Google, DuckDuckGo, Yandex
hornyyy site:rule34.xxx inurl:pool
or Bing
hornyyy site:rule34.xxx instreamset:url:pool