Personally, I'm not going for Mounted Wraiths because of the Hexwraith issue that was brought up. Even as a mistake the uneducated would make the fact that it would be made is... yeah. Half of it is that Mathilde already knows too much about necromancy. She could certainly explain something that looks kinda-sorta like necromancy away, but I'm worried this summon might mean she would have to.
Beyond that, with so many people bringing up the prospect of passing said spell down that also comes into it. Lord Magister Mathilde Weber might be able to explain it away, but would every wizard after her?
--------------
Regardless when this came I up I decided I wanted to pick four broad options that each tied into Mathilde's legend in some way, and this vote has definitely delivered.
[X] [RIDER] The Dämmerlichtreiter
On the minus side being Mathilde on shadowsteed means it can't be passed down. On the plus side, doppelganger utility. On the third side, it's The Dämmerlichtreiter.
Overall I'm fine not passing the spell down if it comes to that.
[X] [RIDER] Orc on a Chonky Boar
The prospect of calling in a Very Lost Orc was hilarious when it first came up, and it remains hilarious now. Also, plausible deniability! Who would expect a Boar Boy to show up out of nowhere and pick a side to fight on? No one. Who would be surprised? Also no one. The fact that it was once a Red Rider will probably only make it faster.
Yeah, this is the option I'm picking because I'm laughing too hard not to, but I think it holds up even past the joke.
[X] [RIDER] Spider
The We would like to play. This seems like something that would start interesting on its own and get much more so in multiples. One spider? Probably quite useful is caves, hallways and in marginal utility countering Big foes. Calling up multiple spiders? Oh no.
[] [RIDER] A man who could look like a depiction of Ranald, though there's a lot of ambiguity about it due to the smoke drifitng around him partially obscuring his face at all times, in a knight costume, wielding swords, riding a horse-sized housecat.
[X] [RIDER] Great Cat Knight
I feel like Ranald would find it humorous.
The fact that the Big Cat steed form in general is also also noted as specifically geared towards fighting monsters is something of actual utility too. I imagine that would probably make it less useful in a general Cavalry role, but the tradeoff of something significantly more useful at protecting friendly forces from giant monsters is interesting.
EDIT: Okay it's been pointed out that this is more anti-single target and anti-Skaven. Not keen on the former, moreso on the latter.
-------------
[X] [SEVIROSCOPE] Auditory
Geiger Counters!
God they're both so tempting. Whichever one we don't pick now I am definitely voting to do later.