UPDATE (11/28/18): D&D Beyond has since updated their online character sheets to include and customize all info about Animal Companions/Familiars/Pets, so this post is kind of moot now. Still, it was fun to come up with ways to homebrew things!
Our problem: Lindsey is currently playing as a halfling beastmaster ranger in our home game, and was getting frustrated trying to manage her Wolf companions stats on note cards during play. Thankfully, we use D&D Beyond, and I devised a way to help her out.
My solution: Create a Homebrew animal companion character sheet she can interact with directly during the game.
First off, I created the Wolf as a playable race, using the stats from the monster manual. Secondly, since you can only create subclasses and not full on custom classes, I developed the Animal Companion subclass for rogue (rationalizing that a Wolf companion would more than likely be akin to a rogue than other classes), which included special level unlocks, like proficiency with armor and saddles to make it more effective as a war mount.
Finally, I customized its signature bite/trip attack maneuver in the combat section, and there you go.
Now, Seth
and I are discussing ways to make her animal companion its own player character almost, with more in depth advancement possibilities.
TL;DR: Don’t feel constrained by the rules of this game. One of the best aspects of D&D are thinking outside the box, and coming up with brand new ways to add extra complexity to the game! Just thought I’d share. Happy adventuring!