Help creating a custom monster

[Sakoyo]

I'm hoping you guys can help me with creating monster.

Here's a little backstory first. I'm DMing an AP and a few sessions ago, the party entered a bathroom on the second floor of a house that was "trapped". The floor was so rotten that the additional weight of a medium creature for a 3 rounds was enough to cause the floor to give and drop the player/s and bathtub into the bathroom directly below it on the first floor. The trap got triggered and the bathtub fell. The party had never checked the bathroom on the first floor before going to the second floor. In the bathtub on the first floor was supposed to be a diseased rat, but since they checked it after the incident with the upstairs bathroom, I determined that the upstairs bathtub fell on the first one and crushed the rat. When the party checked the bathroom and saw what happened, the druid in the party said something about it being the circle of life or the way of nature. Somebody responded with "Yes, the natural predator of the rat is the bathtub". This evolved into a set of jokes from everyone about how bathtubs are the apex predator of the wild and they're known to climb trees to drop on their unsuspecting prey and how there are different varieties, like the Jaquezei, which is a fire type. This gave me the idea of creating a bathtub monster to toss at them at some point when they're travelling.

I haven't sat down and tried to make this yet and I've never created a custom monster before. Does anyone have any helpful tips for creating a monster or ideas on some neat abilities to give the dreaded bathtub?

[Grglspyzx]

I LOVE this! Is this RotRL? It sounds awfully familiar. If so, there are a few places you can produce said bathtub... in book three, there is a scripted event that you could preface with the group seeing rats swimming in terror from something, only to be chased by said bathtub, which, depending on the flavor, you could introduce as a baby of the encounter. (This is me thinking of a floating clawed bathtub, using curtains as appendages)
Also, later in book three, one of the areas they go to has an area known to be filled with the Fay. There is a haunting encounter that is pure flavor, but adds a nice Bermuda Triangle feel to it, linking real world to Golarion. It could maybe be found there, or even in the swamp outside it, maybe followed them out...
As far as creating a bathtub, figure out where they would come across it, then pick creatures that would fit in the area. Reskin it, keeping in mind there are different types of tubs. For instance, for a jacuzzi, it's found on mountainous streams. Reskin a turtle to be larger, does four "breath attacks" of hot water. Lies in wait at apparent watering/bathing areas. Lies below the surface, near the shore, creating a pocket of warm water. Takes away tired/fatigue down one step. However, it takes a will save to get out of the area of influence, with the save DC getting higher each round. If pro oked, it will come out of the water and attack with the jets, dealing damage.

[JDLPF]

I'd simply take the stat block of a medium animated object and call it a day.