The monsters in PS1 range from 01 (Swarm) to 4A (Saccubus). However, since it's 8-bit, it can be forced to read anything from 4B to FF as an "enemy". Many crash the game, but not all, such as this one:
<blockquote class="imgur-embed-pub" lang="en" data-id="a/WKzxL7W"><a href="//imgur.com/a/WKzxL7W"></a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>