I don't think it's a must. But popular utaites have albums and sells keychain and stuff, so they'll need an OC. They also often have choruses with original PVs where they'll need their OC so the illustrators & animators can do their work.
From what I see, some newbie youtaites ususally use fanarts/other people OCs or avatar generators from phone or online. Or if they can draw, they draw it themselves.
If you wanna make one, you can start by drawing it yourself. If you're not an artist, then use avatar generators (currently popular is chanrio, i think). If you want high quality OC pics, you can pay artist to draw (commision). Or if you don't have money, find an artist who wants to draw for you (don't shamelessly ask and create problems).
Don't use arts if you don't have permission. Some artist allow others to use their arts with credit, so please credit if they allow you to. If they don't allow you to use the pic, don't pick fights.
You can always use your real life pic, your pet's picture, or just anime/manga screenshots for a start.
(Zero-chan, google, yahoo, bing, we heart it, instagram are mostly reposter. Don't try to search there. Try pixiv, deviatntart or tumblr. if you're desperate to find a good pic.)