When Diablo 4's Season of the Construct begins, everyone will have a robot spider crawling along beside them. In all the footage Blizzard has released leading up to tomorrow's season launch, the little spider always looks like the Dwarven spiders from Skyrim. Despite what players had been hoping, it will always look like that, Blizzard told PC Gamer.
For the next three months, the Seneschal companion will look like a robot spider and there's unfortunately nothing you can do about that. It won't pick up gold or items like Diablo 3's pet either. The only customization you'll have is for its skills in combat, which is the focus of the season. You can give it skills via Governing Stones and tweak those skills with Tuning Stones, one of which will give you four ranks to all your skills like one of the most popular Uber Unique helmets in the game.
The Seneschal will be Diablo 4's first real companion outside of the NPCs who join you for specific quests. Blizzard hasn't said if it plans on adding companions or pets to the game permanently, but it's rumored to be a feature coming in the first expansion later this year. Season 3's companion could very well be a trial run for that.
I'm not surprised you can't change the look of your Seneschal. A list of transmog items for a companion who will be gone after three months is probably hard to squeeze into a live service release schedule. But it's a little sad that equipping certain stones doesn't at least change its blue glow. All of its skills will change based on how you augment them, so not everyone will have a spider who spits out red beams of fire that chain to every nearby enemy, for example. So even if nobody will have a pink spider at their side, I can at least peek at whatever ridiculous skill combination they come up with.
It's not all sad news though: You can actually pet the Seneschal like you can the dogs in Diablo 4. If you use the hello emote near it, your character will give it a few pats.