Hello, and welcome!
What you describe isn't too uncommon among neutered cats. Even though he is neutered, he could have gotten so excited and stressed (good stress) from holding his favorite toy in his mouth and yowling loudly to get your attention, that his penis made an appearance.
One thing to keep in mind is that if he hasn't shown any sexual behavior since you have him and since he was neutered, we will assume his neutering was a success. However, with some cats, a neuter can be incomplete and if a small amount of testicular tissue remains, it can generate enough testosterone to cause a sexual reaction. In this case, the vet would have to do an x-ray or exploratory surgery to see if any testicular tissue remained and then, remove it. It's also possible that when he was neutered one testicle had not yet descended, so only one was removed. You can have him evaluated in person by his vet, to discuss these possibilities, but if you've had him for a while and/or you know from the neutering paperwork that he was neutered at a young age, (around 6 months or before) and there were no complications, this is most likely not the reason why his penis was out, tonight.
Sometimes, a male cat will do this when he has to urinate and/or is having a problem with urination. Check the litterbox to see if he made urine 'deposits' today and all is well in that department. If he has a urinary tract infection, or is beginning a blockage, this is a medical emergency in male cats. If he's acting normally and not visiting the litterbox often and only a few drops or nothing come out, AND he was looking for attention/activity with you, it doesn't sound like he was in any pain or couldn't urinate, but keep an eye on him. Here is info on urinary problems in cats:
Keep an eye on him to see if this happens again and/or frequently. Also, monitor his food and water intake and elimination. If you notice any unusual symptoms and/or this same thing happens repeatedly, then it's time for an in person vet visit so you can get a diagnosis and treatment, if needed.
I hope all will be well with your furry boy!
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