So good news/bad news here, depending on your outcome. If you currently have your instance of Marketo hooked up to Salesforce, simply create a picklist field (and a second field to hold the text string for "Other") there and have it flow down to Marketo--everything will work exactly as you want it to.
If that's not an option, no worries--you can still create Marketo-only fields and even have the conditional "Other" field you had before. You will, however, lose the ability to prefill this field in the future due to the limitations of Marketo. You simply need to create two string fields and set up a picklist with your form that holds those values.