if with regex - string matches either entire string OR entire string + another character?

Sorry if a question like this already has an answer, I wasn't really sure how to word this, so I'll try my best.

How can I make an if statement match an entire string, OR an entire string with an extra character?

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Let's say the $animals variable has ,dog,cat,pig,bird,goat, stored in it. I want to see if the user-defined variable $your_pet is either one of the comma-separated values OR if it is with an "s" at the end.

So maybe something that would work like:

your_pet=bird

if [[ "$your_pet" == ,*"$animals"*, || "$your_pet"s == ,*"$animals"*, ]]
then
....

However, I need to check if $your_pet ends in an "s" or not, and accept it if it does or doesn't. (The above code is untested but I am almost certain it would not work).

Here are all of the allowed values of $your_pet:

dog
dogs
cat
cats
pig
pigs
bird
birds
goat
goats

And here are some examples of what would not be allowed:

my dog
my dogs
 dog
hamster
hamsters

I feel like using regex in the if statement would do the trick but I don't know that much about regex to know what to use. (if regex is even needed in a situation like this)

Using bash 3.2.57(1)-release on OS X 10.11.6 El Capitan

Thanks