The Android phone is the more likely culprit, since you'd have to go out of your way to set Yahoo up as POP3 on the iPad.
Once upon a time POP3 was the only option for a Yahoo account, unless you had a paid account. Apple swung some kind of deal so that iOS device were IMAP (by default) instead. I'm pretty sure they changed that a couple years ago, and it's possible to use IMAP with any device on a free account.
No idea how to set up Mail on your android phone. There is probably a dedicated Yahoo Mail app for the Android Phone. If Yahoo is your only email account, it would probably be easiest to just use that. Otherwise, you may need to ask advice on an Android forum or Yahoo's support site.
On the off chance that it is a problem on the iPad, make sure you used the default Yahoo option when setting up the account. You can also download the Yahoo! Mail app and see if it works.