Which antialiasing filter before equispaced sampling
Yes, you have to apply another filter before downsampling in order to avoid aliasing. Your original signal has been acquired with a sampling rate $f_\mathrm{s} = 1.2\,\text{kHz}$. Taking every second sample effectively means that you divide the sampling rate by two, so your new sampling rate $f_\mathrm{s}'= f_\mathrm{s}/2 = 600\, \text{Hz}$. To avoid aliasing you have to make sure that the signal contains no frequencies above $f_\mathrm{s}'/2 = 300\, \text{Hz}$ before you apply the downsampling. If it is certain that your signal does not contain any frequencies above $f_\mathrm{s}'/2$ then you don't need a filter.

The anti-aliasing filter can be implemented by an FIR filter. Its properties like stopband attenuation and width of transition band depend on your signal: if you have spectral components near to 300 Hz, the filter needs to be quite steep and it will consequently have a high order. If hardware implementation is an issue, have a look at Cascaded Integrator-comb filters.