Unfortunately Siege Rhino, the value champion, isn't legal in Penny Dreadful. However, we do have Acidic Slime. Slime can solve any non-creature problem. And if there are none of those around, it hits lands. Potentially ending the game via manascrew or card advantage. Between that and Deathtouch? Slime makes a pretty decent Rhino impression valuewise.

So: First we have the original 4 Acidic Slimes. Supplemented with 4 Brutalizer Exarch, 3 Sylvan Primordial, and 3 Mold Shamblers in the side. (14 Slimes)

Then, in classic SaffronOlive style, we have 4 Animate Dead, which can each target a Slime. 4 Stonecloakers, which can each bounce a Slime in response to removal to be replayed, basically making them Slimes, and in the Sideboard we have 2 Oath of Ghouls, which can supply an infinite amount of Slime in creatureless control matchups, but to be fair we'll just call them 2 more Slimes.

And that's it! that brings us to 24 Slimes.